1. Higher 0.625m fuel tanks
I really miss a 0.625m diameter tank higher than the Oscar-B. It would help a great deal reduce part count on small asparagus settings. At the moment you need to stack at least 5 Oscar-B multiplied by x6 symmetry. This really quickly increases part count and reduce frame rate on my not very powerful computer... Even better : include multiple sizes of tank for 0.625 diameter the same as FL-T 100, FL-T 200, FL-T 400 and FL-T 800.
Also intermediate size xenon tanks would be great. Ideally two : diameter 0.625 with two and four times the height of the PB-X150
2. 3.75m fuel tanks/adapters to 2.5m and 1.25m
Would make easier assembling big launchers and reduce part count on take off
3. More Liquid Fuel only tanks and adapters
The same Rockomax and Kerbodyne tanks with only LF in it
Mk-3 to 2.5m regular and slanted, Mk-3 to Mk-2
Maybe a selection by right clicking on the tank in editor to switch between Full LF or LF+Ox not to have a too many tanks in the list
4. More 3.75m diameter elements
Battery, reaction wheel, Probe controller mainly for recoverable booster stages
5. Engines
A larger ion engine for more thrust to make it more usable with crew modules
An in- game electric propeller, not having to build it DIY
A 2.5m engine arround 500-600 kN of Thrust in vacuum and mass around 1.5-2 tons to fill the gap between aerospikes and Vector engines
5. Bigger wings
Square or rectangular wings with an area between 5 and 10 would also reduce part count while making planes
6. Inflatable heat shields
A smaller one to fill the gap between 3.75 and 10m existing heat shields
A bigger one also for big lander to eve sea level
Improvements/Bug fixes on the Jettison option (takes half the craft with the shield whereas a rockomax decoupler provides a clean separation)
7. Command Pods
A 2 seat, 1.25m, reasonably
8. The cracken
Improve the behavior of auto strut making it explode less wanting to invoke the cracked for rapid and explosive disassembly
9. Bigger RTG and bigger parachutes
10. Tourists crew transfer
Either allow them to EVA (in case of emergency/construction mistake with penalty on reputation) or allow to transfer to an external seat
11. Reentry trajectories
There is a mode called Trajectories but I think it should be in the stock game
