Glad you're enjoying the game! If you like this, be sure to check out AdvanceQuest from Button Shy games – it's the same system, licensed to them for a line of card-based RPG's.
As for other people writing content for the system, Button Shy and I agree that it's fine as long as people are just sharing it as personal work, it's offered for free if shared, it includes a credit linking it back to me and the system, and it does not include any racist or otherwise offensive content.
If you wanted to, say, make a series of sci-fi adventures using the Dungeon Hero mechanics then it would just follow the above stipulations. If you wanted to flesh out a larger game with the 2x2 mechanics, that would be a separate issue. Game mechanics cannot be copyrighted or owned, so assuming it was different enough in presentation and description from Dungeon Hero to differentiate it, it would be your own work (although I would certainly appreciate an acknowledgement if it is heavily based on my designs). If we wanted to synergize and make an "ecosystem" of 2x2 games, kind of like what PbtA is doing, then that might require more coordination. For something like that, we'd probably also need to talk to Button Shy as well, though, in case they want a special exclusion for the stuff differentiated by AdvanceQuest.
