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

Realistic commissions that space agencies do. This would be similar to for example, Truck Simulators. Say there's 5 slots. Each has a randomly generated task like "retrieve rocks from [planet] for Droo University" or "get into orbit around [planet] and take pictures for Gobble Maps" or "deliver [amount]kl of fuel to [planet] for SNASA" maybe some unique new missions like "the Hubba telescope is broken, build a new one" and you'd have to build a replacement satellite with antennas, cameras, and a telescope. Maybe a few joke missions like "The Edison Motor Company would like to launch their car into space"

As you complete missions, slots would open up and be replaced with new, random generated tasks that never run out. Also, keeping it randomly generated could keep missions going in custom universes from the upcoming Planet Builder

There's so much potential in generating tasks to do. Just simple formulas of "[action] at [planet] for [name] with a reward of $[distance x task value]" would keep the game going forever.

I loved SimpleRockets 1, but pretending in the sandbox gets boring after a while. Like "I landed, gonna wait here a second, pretend I'm doing something, alright time to go back I guess"

Another small idea I had that I think would be interesting, is that completely at random, at any time, there's a small chance you'll get an end-of-the-world mission. Something like "Droo is ending! Bring as many people as you can to another planet" or "the sun is expanding, you have to leave the solar system" if you win, maybe we can see Jundroo exercise some creativity and have some cool supernova animation of the sun destroying everything, then credits roll. After the credits, the space agency continues from a new planet. From there it would just go back to the endless tasks, maybe one of them being recolonizing a destroyed Droo.