If it weren't for time-warping, yeah, sure, I can see ksp MP having somewhat high player counts, but the game doesn't work without time-warp.
Here's the only way I can see MP working:
One person hosts a game, putting them in a lobby from which they can choose settings, difficulty, mods, and invite 1-5 of their friends. As they join, they appear in a list with a "ready up" checkbox and (if it will be possible to play as separate space programs instead of the same one,) a "space program" option next to their names. (Players can create their own space program with a flag, default colors, and Space center location, or they can choose to play in the same space program as another player). once everyone clicks "ready up" the game will start.
Players in the same space program share money, tech, missions, etc. Players in separate space programs have all their own stuff, but are not necessarily competing, and can always cooperate.
Time warping is up to a unanimous vote, but once begun, can be stopped at any time by anyone or by the Kerbal alarm clock. Reverting to a previous save is also up to a unanimous vote.
If a player stops a timewarp to perform a maneuver of any kind, other players have the option to spectate them or request to co-pilot the vehicle.
Time passes even when a player in the VAB or similar menus.
At no point should the game desync players and make them manually resync with each other like how the ksp 1 MP mods work. every player is always synced up. The way I see it, if everyone is out of sync then there's no reason to play MP.
Any number of players can quit at any point as long as every space program still has at least one player in game.
If a space program has no players left in game, the remaining players are given the following options:
1: Save and log off.
2: Pause and wait for the player to return (if they crashed or lost connection, for example)
3: Switch to play as the empty space program (as long as that doesn't leave their previous program empty)
4: Permanently integrate the empty space program into one of their own.
5: Permanently delete the empty space program.
When the players want to play again, anyone who saved the game can host it again, inviting their friends back into the lobby.
Just as before, the host can change settings, difficulty, and add or remove mods.
The players all appear in a list with a "ready up" checkbox and a "space program" option next to their names.
Once everyone clicks "ready up" the game will resume, unless one of the existing space programs is empty, in which case the players are given similar options:
1: One or more players choose to play as the empty space program (as long as that doesn't leave their previous program empty)
2: Permanently integrate the empty space program into one of their own.
3: Permanently delete the empty space program.
If a new player wants to join, they can either join an existing space program or create their own new one, in which case they are given the average tech and money of all the other space programs.