Hmm...well my opinion isn't worth as much, since I've only played IV, TMS, Persona 4, and the Devil Survivor remakes (and part of IV:A), but what I'd like the most are:
-Ability to save anytime: This is the most important to me. I played the vanilla version of P4...and the saving system honestly soured my experience overall. Being forced to replay the same two hours, just because I got unlucky after a long dungeon sequence, is not a challenge. It's purely tedious and annoying. I'm really glad TMS and SMT IV fixed this...and I hope I don't have to see it again. Being limited to saving in certain locations is honestly a mechanic I wouldn't mind going extinct.
-Diverse party members: This might not be for everyone...but I enjoyed Persona and Devil Survivor a lot more due to an interesting cast. While I enjoyed the tone of IV, there was honestly only one character I really cared about...which is a big problem in a game with such an interesting story. I would see these tragic moments happening being the cast and...just not really care. IV:A improved on this somewhat...at least based on where I currently am.
-Ability to control what skills are inherited. Maybe just a personal thing...but while playing P4...I remember going in and out of the fusion screen until I would get the skill combo I needed. I don't find tediousness to be fun, and so I hope the option to inherit skills remains the same as SMT IV and the Devil Survivor games.
The tone and difficulty are both fine to me whatever they choose, as the games I've played have covered a fairly wide range over both aspects. Whatever they plan to do...I just hope it's well done.