Add a campaign mode
You should make a campaign mode, for example: you have a certain amount of money to build a ship. Then you need to complete different objectives and missions (such as: achieve orbit on smearth, or: land on the smoon) to get more money to build better ships.
The main goal would be to build a space station where you can refuel your ships and research new parts.
Or you could try to build a space station on the smoon.
Something like land on moon to unlock landing legs escape smearth to unlock nuclearn engines and etc
I think with science will be better (like campaign mode in KSP)
Geoff Canyon commented
For campaign mode wouldn't it be a workable first step to:
1. add habitat and supply modules as available parts
2. make placements on the various planets permanent
3. allow rebuilding/consolidating parts and fuel on the various planets to create/launch new rockets from there.
With just the above we could build and supply a Smoon base, ferry raw materials/parts to it, build new rockets at it, launch them from the lower gravity well there to the other inner planets, then even on to the outer planets.
The competition on the leaderboard to build/supply the largest base on Smeptune would be intense.
Rick Kyle commented
An additional Idea for this could be a parts restriction in campaign mode on all planets but Smearth. To Unlock, you need to land X number of parts in a designated 'drop zone' on the planet... or alternatively X number of Fuselage components... maybe a specific module for the campaign called a 'colony pod' must arrive first under time restriction, and a resupply module that must be landed using less than X amount of fuel to build there...
The end goal would be to colonise every planet...
Research could unlock by completing:
1 send orbital probe,
2 land a probe,
3 land a rover,
4 land smearthlings/colonise,
5 return smearthlings,
Just some thoughts...
Becca Horsfall commented
Not everyone is a rocket scientist, some more missions that help to explain things a little better would be nice. Ideas on rocket designs etc.