@Wonder
Really depends on the mood we will want for any of the areas, and scenarios.
As for the gameplay I think a old "Metroidvania" style gameplay would be awesome, you know like how Castlevania Symphony of the Night, Aria of Sorrow, and Portrait of Ruin (and others). I think that style of gameplay would probably be best. I know it's done before, but other than an older turn based RPG style a Metroidvania style sidescroller would probably be a good option.
Also if we were to make a Multiplayer versus... that would be interesting, but it would also be interesting if we added a Multiplayer co-op. But we would need to find a way to add multiplayer support for it.
But yeah as for the music I would say it would be hard to think of. 16 bit music can be epic, and dramatic.
For example the song Laughing Mad from the end of FFVI from the snes.
https://www.youtube.com/watch?v=JbXVNKtmWnc