My ideal would involve a few things:
1) I'd like them to continue following the original story as much as possible while evolving the plot with Aeris (she'll always be Aeris to me) a bit more. She's always been my favorite character, so I'd really like to explore her background and the background of the Cetra a lot more than we had the chance to do in the original (which wasn't much.)
I want them to build solid back stories of all the characters, but most of all with Aeris.
2) I want there to be secrets and mysteries Lots and lots and LOTS of them. In the original you had rare materia, chocobo breeding, the weapons, key to midgar, Lucrecia, the traveler in Kalm, etc which were off the path of the main story and gave the game such depth and made you want to keep on playing it over and over again. Games of today have lost that depth.. most of the time there's no reason to wander off the path of A to B, and as a result things feel scripted and shallow. Aside from the story and characters I want secrets and mysteries to be a driving factor of me coming back to play this over and over again. If they do that it will fulfill most of the dreams and hopes I have for this game.
3) I want the battle with Sephiroth to be 100x more epic than the original. Although I loved his angelic form battle it was far too easy. Make him an absolute pain in the &#^ to beat this time around. I wouldn't mind if it took an hour or longer. Preferably it would. Also, I never cared for the Bizarro Sephiroth form, so if they revised that or replaced it altogether with something else I wouldn't lose sleep over it.
Seeing how much effort they're putting into earlier boss battles gives me hope with Sephiroth that it will be something truly epic. Don't disappoint us!
NIGHTMARE SCENARIO:
This thing drags on too long and the current team either disbands or loses interest in the project. Kitase, Nomura and Uematsu are a dream team, and I hope they can see this thing through until the end and do it justice.