Celebrations is a proper idea, and a good one at that.
If they could implement individual players with individual celebrations, I'd actually watch them, instead of X'ing through it all.
They could make a big database of all of the celebrations, that are common.
Heskey & Co's DJ-ing one (the one that's showing respect to DJ Spoony)
The Asprilla cartwheel-plus-gun-ish-action.
Robbie Keane's Cartwheel and Roll.
Montella & Co's aeroplane (already in, but I'm talking about being able to assign them)
Senegal's team dancing.
Bebeto's rocking-the-baby celebrating.
(as mentioned) Shearer's hand-in-the-air.
Ryan Giggs' I-don't-celebrate celebration.
(as mentioned) Klinsmann dive.
Speaking of diving,
surely you should be able to dive during the game? Implemented properly, this would be the biggest addition to the actual gameplay that isn't there yet, that I can think of. Make it easier to dive as Totti etc.
*on with the celebration list*
Certain players (like Bowyer) celebrating over by the opposition fans, to get them angrier at him. Then they'd boo him every time he gets the ball (like that doesn't happen already IRL?).
Definitely, crowd and player interaction would be nice. You make a bad tackle, the opposition crowd punish you for it. Etc.