I made a comment earlier, but it was rather short and I want to elaborate a bit more.
Isometric - Not just for the sake of nostalgia, but for the fact it gives a more tactical feel to the game. Controlling your party, picking out enemies, determining movement etc is all easier and more fluid from the isometric view. Following from that...
Parties - One of the biggest problems I've had with recent RPGs is the fact that it's always only about the PC. Being able to have a sizeable party (six or more is a good number, 3 or 4 is irritatingly small if you have any decent amount of NPC followers) is a must, as is being able to control them in combat, and control their improvement.
Turn-based - more strategic. We have no shortage of real-time games to play, how about letting those of us who enjoy taking their time. I don't even care if it has a real-time mode as well, so long as it has a proper turn-based mode (though hopefully one better balanced than, say, Fallout Tactics' implementation).
Character Customisation - I understand that there's been a recent trend to streamline character customisation, but I'm the sort of gamer who enjoys this part. I also don't just mean customisation in terms of combat abilities.
Good story - an absolute must for obvious reasons. Anything inspired by Planescape here would be brilliant.
Settings - I'd enjoy a cyberpunk or near-future setting personally, but if it's fantasy at least try to avoid plain high fantasy or dark fantasy. You can make high magic fantasy without making it elves, dwarves, orcs, dragons and humans. And low magic fantasy doesn't need to be grimdark.