Sounds fair enough for a basic premise, and you got my interest [i]if[/i] extensive knowledge on the SMT games isn't necessary. I've played the Persona games, the first Devil Survivor and Digital Devil Saga and been part of an SMT rp in the past - but that's about it. 

So with that, I'm also not sure how much help I would be in fleshing things out with my limited knowledge, much as I'd like to help. 

Only random suggestion I could throw off the top of my head is that perhaps there could be seven protagonists (a good number of players for a small-mid group), with each one being tied to one of the seven deadly sins? Perhaps they could contract demons tied to that sin better, or attract demons of that sin in general (both in good and bad) towards them? With perhaps the Seven Princes of Hell being bosses they need to defeat, with the corresponding protagonist getting to shine when facing a prince of their own branch of sin?

Cliché, but y'know.