Character diversity is really not a problem with Final Fantasy. Unless you're just saying they need different races.
In terms of just very different characters, that's kind what they've always typically done anyway. Look at FF7 and FF9. They almost deliberately design the characters to cover all the different bases. Almost unrealistically so. You never find a group of such different people hanging around together like that in reality.
FF15 (and FFX-2) are perhaps the only games where the party is a realistic composition (i.e 3 girls going around together or 4 guys going around together is something that actually happens. You very rarely see a young child hanging out with a teenage girl and an old man ala FF4)