What's up with the specific submission rule? Metroidvania game jam is also running and I would like to make a game mixing boh genres, this are usually extensive games so why can't I make a game and submit for both jams?
This is mostly to prevent spam entries and ensure that games made for this jam were actually created for this jam. You're free to participate in both jams but you must have a separate game entry for RPG Mania. RPG Mania also now runs twice as long as MVM so if you manage your time properly you should be able to do both, depending on the scope of your project. I apologize if this rule rubs you the wrong way but it was prompted by numerous spam entry submissions in the first RPGM. This is why we can't have nice things.
