I've been playing KSP for the last few months and I think it's fantastic- the combination of sandbox-type construction and fairly realistic physics is great and I eagerly await the release of the sequel. Until KSP2 comes out, though, there are a few things that I'd like to see in the current game that would make a big difference without requiring a massive amount of work. I work in a software development team so I know something that seems 'easy' rarely is, but I've kept the list to fairly small changes or additions which are based on existing features to keep things reasonably simple.
1) Improved fuel tank paints.
The standard 1.25m tanks have four different colour schemes, but one of those is plain white and the others don't match up particularly well (especially the little FL100 tank and its strange black stripes). The 2.5m Rockomax tanks only have two colour schemes to pick from- a black and white style that for the second-largest means white with a big corrugated stripe in the middle, or orange which does the corrugated thing to the second smallest tank instead. The Kerbodyne 3.75m and MH-specific 5m tanks have one colour scheme- ONE!- and it's a bit boring. The little 0.625m Oscar-B tank is likewise stuck with one boring grey paint job, and the surface-mounted orange tanks also have one colour scheme to share.
The Making History 1.875m tanks are another story altogether- they don't even match each other! The A150 0.625m-1.875m adapter seems to use the same orange as the Rockomax tanks for some reason, which is a different shade to the other 1.875m tanks which is different to the 1.25m tank orange; the 'white' patterns aren't white (in some cases there's more black in the white pattern than the black and white one!) and some of the black and white patterns are visibly more grey than white compared to others, and don't match the other stock parts at all and what is with the A151 fuel tank having a thick black vertical stripe when none of the other tanks do? In addition, several of the 1.875m engines such as the cheetah or bobcat have bases that work fine with the 'white' versions of the two smallest tanks but are terrible with the other colours and tanks, especially orange! Clearly a lot of the tank designs are meant to look like old NASA rockets such as Mercury, Delta and Saturn or the Space Shuttle launch system, but this leaves very little scope for building anything else.
Instead of this mishmash of paint schemes, I'd like to see a custom paint system that would let you pick from a range of different patterns including vertical and horizontal stripes of varying sizes, new patterns based on real rockets (there are a lot to choose from- Russia/USSR have plenty of rockets to pick from and India, China, UK and Europe- no political statements intended, I just like the Black Arrow's spiral paint- all have options too) as well as textures like the corrugated or tile-like patterns that already exist or even something totally new like polka dots or geometric dazzle camouflage, then choose the frequency of that pattern (how many stripes do you want? etc.) and then the colours using RGB sliders- I'd say stick to two or at most three options or the whole thing could get too complicated.
Applying the pattern consistently across different sizes and shapes of parts might be tricky, but the result would be great to look at, especially on the Steam workshop and sharing ship designs/screenshots on the forums. Serial creators could use their own individual patterns to make all their stuff look the same rather than everything being white, you could use different paint patterns to mark specific vessels depending on what they're for/where they're going etc. and for pure creativity it would be a massive bonus. Being able to save a particular pattern and apply it to multiple parts on the same vessel at once and on new vessels would be required to make this feature useful- doing each tank individually would take so long that it wouldn't be worth it.
This system could also be applied to the payload fairings which are similarly lacking in aesthetic appeal and which would also benefit from custom painting.
As a quick fix, how about making the grey and orange patterns for all the different tanks use the same grey and the same orange? This is particularly applicable to the making history parts where there are several different shades of orange in use on different parts that don't match each other or the stock 1.25m and 2.5m tanks. I actually prefer the MH-style white with black highlights pattern to the 'dark' theme for 1.25m tanks, but they should all use the same white and the patterns should match up more closely. The 'white' and 'black and white' names for those tanks should probably be reversed as in most cases the black and white version is mostly white with a few small black bits whereas the white version often has large black patterns on it.
2) Docking alignment indicator in docking mode.
I'm surprised this isn't a stock feature as docking is a pretty important part of building anything in space and is also quite hard to do (trying to keep the two ships aligned using target tracking will in some cases inexplicably target the wrong part so the ports keep moving away at close range, and target tracking isn't available in SAS without a highly skilled pilot or a later-game probe core/part in Career mode). A simple docking alignment mod is out there and is incredibly useful so a stock implementation that ties in with "docking mode" and shows where both docking ports are pointing and your direction of movement seems like a quick win. Another related beneficial feature would be the option of locking docking ports to set angles e.g. 90 degrees of roll relative to each other, or the ability to rotate sections once they've docked; either one would make building larger structures in space considerably easier.
3) Tracking station upgrades.
I'm sure most people have done it: launch a mission to one planet/moon/whatever, go and do something else while it's on its way, then realise you've missed the planned insertion burn and your mission has been launched out of the target body's sphere of influence and potentially right out of the solar system itself, with no way to get it back without loading a previous save which could potentially undo several hours of gameplay. A warning of some kind in the tracking station, or even better when time warping anywhere including on other vessels, when something is getting close to a maneuver node would make it much harder to miss an important burn to put a probe into orbit, correct course or just pay attention to a ship as it's about to re-enter on Kerbin. Sure, there's a mod that can do something similar but you still have to set the alerts manually so if you forget or get it wrong, you're still going to miss that node.
Also in the tracking station, I'd like the ability to rename a vessel and change its type without having to load it up. That could prove useful for something that has a shroud on it (I usually keep the shroud on for missions to Eve/Jool to better protect the rover/lander inside when aerobraking into orbit or to the surface) and save time if you've launched a load of missions with sequential names only to notice that there are two 3s or you've missed 11 and need to change a lot of names to correct the problem. Another useful feature would be to show remaining delta-V in the tracking station which would be useful for seeing if those 'move this vessel to a new orbit' missions are feasible. It would also be helpful to see the total required delta-V for all planned maneuvers in the tracking station and on the map as you're flying, so you know if the set of precise orbital maneuvers you have planned out will run out of fuel half way through (we've all done it, right? ...right?)
4) Aircraft improvements.
The Mk2 cargo bay feels very cramped and doesn't work particularly well as a rocket-launching plane because you can't fit anything in except a 1.25m fuel tank and possibly some of the cheapy fins if you shift them inwards a bit and put them only on the sides. If the Mk2 parts were slightly bigger (maybe 10-20%) then there would be much more room to fit more useful parts inside and make using a plane as an alternative to a first stage more feasible.
There's an obvious gap in the array of wing options too- there's no square Big-S wing, only a triangular one which isn't nearly big enough when used with the other Mk3 Space Shuttle-style parts. A square wing would make large aircraft fly, and look, much better than using the much smaller square/rectangular wing pieces. There's also an opening for a Mk3 inline docking module with the option of a regular or large Clamp-O-Tron docking port and possibly a Mk2 rear-loading cargo ramp to match its Mk3 counterpart.
Something that could prove particularly useful on planets other than Kerbin is an electric-powered aircraft engine. It doesn't have to be massively powerful- something a similar size and power to the Juno jet engine would be sufficient- but being able to run on electricity would allow aircraft to operate where the atmosphere doesn't allow conventional engines to work. It would also have the benefit of being able to refuel using solar panels, fuel cells or RTGs meaning it could operate for much longer than a traditionally-fueled aircraft could.
5) Expanded deployable science options (for Breaking Ground)
I only recently downloaded the Breaking Ground expansion but the deployed science array seems rather limited. There aren't that many experiments to use- mystery goo, weather, ionography and seismic- and they only work half the time because there are no batteries to store charge from the solar panels (the RTGs are at the very far end of the career tech tree). They also require a crewed mission to deploy them in the first place and generate science very slowly even when deployed by a 2-star scientist, which is the best you can do without venturing beyond Kerbin's orbit. I'd like to see more experiments such as a ground sample analyser with its own little sample collection rover, a telescope/observatory of some kind (better results with no atmosphere) and some kind of particle collector that catches and analyses dust (better results in atmosphere because wind will carry the dust but still functional without an atmosphere because dust gets thrown up by impacts). There's a lot of scope for expansion here!
There's also the option of having a larger unmanned experiment control core that can deploy the modules by itself, much like the InSight lander deployed a set of experiment pods on Mars. This would obviously have a limited capacity and limited range of deployment meaning there wouldn't be much space to deploy the modules around the core and lower science generation rates than modules deployed by a Scientist, but the central core would have some power generation (enough to power at least 2 deployed modules should do) and communication (similar to the HG-48 module) abilities of its own and would be ideal as an advance mission which could then be expanded by a crewed mission adding extra modules later.
One last thing- when trying to move maneuver nodes I keep ending up accidentally changing the radial in/out values instead because it can be hard to click on the central hub of the node, so moving the adjustment handles out a little bit more or making it one click to move, click again to adjust would be a simple but useful change. It would also be nice to have the ability to have the nodes snap and lock to the apoapsis/periapsis or ascending/descending nodes of an orbit rather than trying to make fine adjustments as the viewpoint constantly changes due to the movement of the vessel and/or planet.