My suggestions

[hazarada]

Fuel:

Instead of fixed size tanks how about you select a tank diameter and then click and drag the height which then determines the amount of fuel. Also an option to enter the amount of fuel and get the tank height created that way around. There could be different types of tanks like ones meant for least dry mass or for impact tolerance or whatever.

Also i'd like to see an option to build a craft with empty or partially filled tanks (they could be created full by default).

Also more control over fuel flow, the one way fuel lines often give unexpected and undesired fuel flow. What would be more functional is a two way fuel line and all tanks connected could have a priority value as to which one gets emptied first and which one last.

I'm aware there are mods for this but an official version of harvestable fuel would be great.

Engines:

In addition to 1 piece engines, what would be cool is separate engine modules. Lets say 3 modules per a functional engine with various options for exhaust, combustion chamber and a mainfold of sorts. Each of them would affect the engine performance and other attributes in some way based on the engine diameter.

UI:

An optional readout of your current periapsis and apoapsis in the staging view would be great.

In docking mode a sort of overlay projection of your current trajectory (like a line) as well as the projected position(like a big X marker) of the closest docking port on the other vessel at the closest point of approach by the projected line

In flight function assignment, sorta like in VAB.

Other:

More wheel variations, faster and stronger ones!

In flight strut construction, my giant space station needs them :/ i'd gladly sacrifice assign many of my kerbonauts to the building task.

[Frederf]

1. Procedural tank parts would be AMAAAAAAAAAAAAZING!

The fuel flow works on a constant fractional rate system. It takes the same time for per percent which means pushing fuel from a small tank to a big one takes a different amount of time than pulling.

2. Build-an-engine would be interesting. I think that would be a kind of open a dialog and do some editing and the part would come out. More actual parts with physics calculations is just going to slow down the game both gameplay and CPU performance-wise.

3. Indeed, actions-on-the-fly mod should be a stock feature.

[Ziff]

The idea of variable length fuel tanks has been raised several times, and I know Harvester commented about it a long time ago. I can't remember if it was an issue with the way parts are handled or created but there were several concerns about them. I can't seem to find the thread at the moment but I will search some more.