The game could be made open source and multiplayer servers placed behind a subscription like many gaming companies are doing now. I'd pay a SMALL monthly fee to continue to have access to this game, in fact I just got vip again purely to support the game and several other people have it as well. Maybe a certain number of games per month are free, then to continue playing as much as you want, throw a couple bucks to Nabi? I realize it's always been free to play, but that model may not work out for much longer. Making it open source would allow people who are passionate about the game to develop it further or patch any exploits that might pop up.
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I guess that would make it not fully open source but you get what I mean let's not get caught up in semantics