Essentialy im asking for pretty much everything to be made customizable in some way. This would hugely expand the possiblities of what you could make, and would make the game so much more interesting. My main criticism of KSP was that you are building from pre-made parts, rather than making your own parts, which hugely limits what you can do. Here are a few examples of what could be made customizable:

Engines
-Already suggested, so ill be brief. Adjustable nozzle geometry, aerospike customization, adjustable combustion chamber size and fuel air mix.

Solar panels
Ability to have panels on both sides or panel on one side with radiator on the other. Adjust how it folds/retracts

Pods
Adjustable geometry, reaction wheel size options, crew slots, heat shield customization (how much ablator it has, whether its detachable or not), options for inflatable rubber rings/balloons (for water landings)

Wings/control surfaces
-basically whats in simpleplanes

Fuel tanks
-Fuel/oxidiser ratio, composition (monopropellent, LF/O, etc)

RTGs
-nuclear fuel type (fuel with a long half life gives little power over a long time, wheras fuel with a short half life gives lots of power over a short time

Antennas/dishes
-Adjustable geometry, change how it folds in/out

Basically, the game should allow you to select what type of component you want, then you build it yourself. There could possibly be an option for saving/loading custom parts