Some ideas about game progression
I'm sure some aspects of this have been discussed, but I wanted to get some of my own thoughts down after talking about it in a recent thread. KSP based its progression around contracts. This worked fine enough. You are given a contact to put a kerbal in orbit, you do it, and get money. This however leaves something to be desired in my estimation, both gameplay wise and realism wise. My proposal - have more granular milestone steps.
Programs
The game progresses with a tree based progression. Each node on the tree is a program - think of Mercury, Gemini, Apollo, Ranger, Mariner, etc. When you complete a program, you unlock following programs, and sometimes branching paths. Each path follows a general focus, such as crewed spaceflight, earth orbit satellites, and deep space probes. You can work on multiple programs at once, and they could even benefit each other, like a lunar orbiter scouting crewed landing sites.
Missions
Each Program is made of individual missions (Explorer-1, Explorer-2). Each mission brings you closer to the ultimate goal of the program. For the first crew spaceflight program, you might conduct a launch escape system test > Launch an uncrewed suborbital mission > Then a crewed one > Fly an uncrewed orbital mission testing out your spacecraft, and demonstrate full control in orbit and can support an astronaut for a day. After you finish the final mission of the program, in this case a crewed orbital mission, you receive a big reward and unlock the next program(s). You'll also be given the opportunity to fly bonus missions to accomplish new tasks: stay in orbit for longer, achieve a more precise splashdown, etc.
Customization
While progressing through the game, you get options to customize your playthrough. With each program, you get to name it, choose mission logos (maybe even draw basic ones in-game) , and name individual missions. Said customizations could then appear on spacecraft and other things in-game. Players would also have the opportunity to create custom programs with custom missions, allowing organization of missions not fulfilling existing tasks.
History Page
Following your flights, you can go to a space agency history page, where it lists your space agency's past programs and missions. Each mission will be summarized, with important stats listed, including which astronauts flew on each mission. The page would also have a filter function, allowing you to see all missions to a specific location, or something like what astronauts have the most flight experience.
Multiplayer
This setup could help multiplayer. You could have a player with another space agency, and you link your program for a crewed lunar landing together - Then one player provides one aspect of the mission, like the primary crew vehicle, and the other the lunar lander. Then once the final mission in the program is completed both players complete the program in the progression tree, and some division of the completion reward. Players could play cooperatively within the same agency, with one player conducting crewed spaceflight missions, and the other doing planetary science.
Competitive multiplayer could be about competing to get to each milestone. When a space agency reaches a milestone, it could reduce the scientific reward other players get for that milestone. This would be slightly mitigated by giving each other player a small bit of science, as though the other space agency is sharing their findings - but not enough to replace what the science reward would've been. The players can offset this though by say, landing on the Moon in a more risky, but scientifically interesting place - or they can specialize in an area of progression the other agencies aren't focusing on. It would thus allow for various strategies, and give a sense of urgency.
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Ultimately this system accomplishes more realistic and more gradual progression that has you test individual systems before going to big missions, and the mission customization tools give you some fun personalization to make each playthrough feel more unique to you. These are only my rudimentary thoughts, and I'm sure there's a lot to improve and expand upon. I hope that the devs are considering similar ideas to this and will consider my suggestions as well.