Hello, I have an idea for a game:
It would be just like a traditional Real-Time Strategy game, but with a twist: the user is given the ability to write AI for their units. There would be a a development mode of the game where you could access basic unit functionality (like move here, move there, attack, etc.), and basic information (like type and position of enemy units you have detected), and write your own AI.
I think this would be really cool , both for people who already know how to code, and for people just starting to learn.
It occurs to me that it would be waaaay easier for me to modify an existing open source game to add this functionality than it would be for me to write my own RTS game from stratch. Like probably the difference between me being able to do it and not (I am doing this on the side of my day job). However, I think it would be cool to at least try to make money from this game.
So I was looking into licensing, and it looks like almost all open source games are released under the GNU GPL license, meaning that they are legal to modify and redistribute, even for money, provided that the mod is also released under GNU GPL, and provides the source code. This makes sense, but it of course makes it difficult to make money selling a game, because anyone who buys it can then redistribute it for free if they choose.
I also read about the GNU Limited GPL (LGPL), which looks like closer to what I want--open source stuff that can be included in proprietary works. You don't have to provide the source code for your modification. It looks like you have to clearly separate your work from the LGPL work by using them as dynamically linked libraries, or something like that. I'm not sure how technologically viable that is for my idea yet. I tried to find RTS games made with this license. I might have found one (Evolution RTS: github.com/EvolutionRTS/Evolution-RTS/blob/master/license.txt), but it's ambiguous, and I'm unsure. Generally though, it seems like LGPL might be more of a thing for tools like game engines rather than whole games.
One idea I have is to modify a regular GPL game, then host it with servers for competitive play on some site like steam. I of course wouldn't advertise that the code is open source, but people would probably be able to get the game itself for free in this way if they looked into it. However, they wouldn't be able to play it competitively against other people without paying on steam.
Any advice? I'd be happy to hear feedback on the legal issue and/or the idea in general.