Gladiator is a must make must have for me. Loved the movie very much, the soundtrack is top notch.
I'd like to see this on a double disc. With PIP commentary and bluescreen for many of the battle scenes. Dolby TrueHD. (Homer like salivating).
The other given to me is Lord of the Rings. And it would have to be the extended version. Again Lossless audio would be a must and some kick ass special HD Features.
Shrek would be another release I would like to see, maybe with a PIP of the cast recording their lines or something.
There are very few titles that I am really interested in replacing in HD though. Of the ones I have, the following would be rebuys for me:
Star Wars Prequels (especially #3)
Jurassic Park (maybe)
War of the Worlds
Custom Home Theater Sales Specialist.