Kitten Space Agency is a space flight simulator in development meant to be the spiritual successor to Kerbal Space Program. KSP was meant have a sequel but Take-Two Interactive fudged it: KSP 2 is controversially still in early access on steam as its studio is all but shut down.
I have high hopes for KSA as it is being made by experienced developers, notably Dean Hall (creator of DayZ) and his studio RocketWerkz (Stationeer, Icarus) who are creating a custom engine with veteran KSP modders being part of the team.
In an interesting and quite controversial move, they are aiming to make the game free, relying on donations and grants from space agencies, and are refusing to release the game on steam (among other things because steam is pushing gambling on gamers).
Their development process is quite transparent trough their disc0rd and they routinely share their advancements: in vidrel you can see a flyby of mars.