So I'm not here to be begging for multiplayer or to tell you what to add. But these are ideas I feel a lot of people would love to have added to the game.

1) Right now, modding is very limited. All you can do is add new parts, and even that, we can't do whatever we want with, just add "new" pre-existing parts. In the next version of SR, there should be an enhancement to modding capabilities. Not only would this greatly increase the community, but others could give you inspiration to what to add to the game and what to leave as mods. So letting us create our own fuel lines, making very custom parts, even our own GUI for new things.
2) As a lot of others have mentioned, adding a playable character would add a new element to the game. You could set goals and missions using this character. And each pod should be able to carry a certain amount of "players."
3) Contruction Bases: maybe not so simple as just launching it up and then that becoming a new "planet" to launch from. Maybe add challenge too it. Such as: flying the "station part" up and then adding "resources," or a custom fuel, to it, and the more resources you have at the station, the larger your craft can be. Sort of adds a certain difficulty element to it.
4) A more organized game. The GUI, to me, seems a bit too clunky. Everything on the mobile version is right next to each other and even moving my finger slightly, effects something else. So maybe not make it smaller, but add "locks" to the items your changing or something like that.
5) More parts would be lovely. Bigger editions of the current parts or something like that.
6) More features to the editor. Things being: Center of Mass, Center of Thrust, Sub-Assemblies, etc.
This would add a lot of functionality to the editor. Placing down the RCS Thrusters is more trial and error and luck.
7) Easter Eggs throughout the game. Something that makes the player yearn to want to explore every nook and cranny.
8) There is very little editablity in SimpleRockets. Two features that kind of co-enside are: Multiple connections and fuel transfers. I'm going to take this straight from KSP and say, I like your struts, but I feel there should also be the "click-and-connect" strut like there is in KSP. As well, there should be a fuel transfer "strut" and you should also be given the option to transfer fuel without being docked.

That is all, thank you for the read.