I see. Maybe this limitation could be turned into an advantage: It sounds like you are a 1 person show (or small team) and still created this unbelievable piece of software. Why not broaden your business model and offer a service where you / your team (maybe this could generate revenues to grow the team) will upload user maps and compile them into the game at a reasonable fee, then offer the upgraded game version to the customer base who purchased the game (myself, I'd be happy to pay some extra bucks for regular add-on maps as well). Anyway this still means you'd have to find ways of chopping the game into multiple versions with different map packs along the base maps, in order to keep the download size at reasonable levels once maps will be added...
What do you think? I'm still trying to find out if a similar mechanism like the Google maps plugin for Unity could help to simply 'load' individual maps into the core game...
There are lots of interesting ideas you have there. It's true that the project could be expanded a lot. However I prefer to keep it nice and simple, and also keep all creative control myself. So my intention for the moment is to keep it as more of a fun hobby project that I can work on in my spare time.
Thanks for the links to the level editor assets! I had't seen them before, you're right that the first two seems most promising. I will look into this more when I have the time.