Great idea, but how would the warp system work. Would you have to agree on a warp length? It would work ok with a few freands but could never be able to have any more than 5.

A local multiplayer with re-dockable ships would be amazing, because huge journeys or challenges could be undertaken by two people at once . Also, reclaiming fuel would make interplanetary return voyages so much easier. You may even be able to escape Smenus' atmosphere using this kind of technology. In addition, multiple command pods could be useful for an Apollo-like return voyage, where less fuel needs to be schlepped to a planet to make it back alive

Two words: Space war

I would honestly love to have a war, in space, with rockets
Although new weapons and defenses would be appreciated

I can see why people would think "it's not like we are gonna battle our friends" or something like that, but we can race them to the moon, around a planet, or even try to dock each other. Or (lol) try to smash each other out of orbit with strut armor plating! Anything can happen :)

I understand I would be difficult-at-best to do this on the scale of having dedicated servers for the game because there would simply be to many rockets. As for same-device multiplayer... the screen would just be to crowded for even 2 players.
However, I see no reason not to implement the ability to have multiple players in a single mission / session on the player's Local Network. Besides, making it multiplayer, even if it's only on the Local Network your connected to, would make the game that much more awesome because then everyone would be able to play multiplayer with their friends.
I know it will take some time and you'll probably need to implement safeguards to keep people from blowing each other's ships up before they're even out of the launch zone, as well as some Observer features and probably some other stuff. But come on! Pretty much everyone wants multiplayer for SimpleRockets. Why not at least give them LAN multiplayer capabilities?

Have a race mode like low flyer where you race opponents ghost ships or your own top score in timed races. eg: 5 min to travel as far along the surface of any planet as you can or just as far from the launch point as you can in the time!