I think, unless it's a huge epic film that dwarfs the extended cuts of the LOTR films, there are too many things we want to see that simply won't fit. I'm trying to stay somewhat spoiler-free, but here's what I want to see in this movie:
The birth of the twins
The revelation of Darth Sidious
The death of Count Dooku
The exile of Obi-Wan and Yoda
The Jedi Purge, including fitting deaths for my favorite doomed Jedi (Mace, Kit, Saessee, Plo, Aayla, and Ki-Adi)
A swordfight to end all swordfights, set above the swirling magma of a lava planet
The death of Jar Jar Binks
Enough background info on Bail so not just us hardcore fans will know exactly who he is
Mon Mothma, Jan Dodonna, Garm bel Iblis, Sate Pestage, and possibly Wilhuff Tarkin
A concrete answer as to why the droids know nothing in ANH and beyond
Whether Stormtroopers are clones or recruits
A scene involving Yoda, Obi-Wan, and Sidious that reveals why they're so scared of him
Lots of Teek, most likely fighting mouse droids
What are midichlorions?
Why did Qui-Gon speak in AOTC?
Why can some Jedi vanish?
What does "Balance to the Force" mean?
What is the prophecy?
What is the Son of Suns?
What species is Yoda? Are there more?
Why was Luke considered safe with Anakin's stepbrother?
What was the final straw to turn Anakin against his friends?
Sidious kidnapping a red-haired baby girl to act as his "Hand" certainly wouldn't hurt
If it answers all these (and more, I'm sure), it can be the best. But I doubt it will. I'm voting "on par with the prequels," since I expect it to be better than TPM but I doubt it will match AOTC.
Edit: Tycho, I've got to disagree with you about how much TPM set up. Everything that was revealed in that movie could have been worked in as dialogue in AOTC, particularly when Anakin and Obi-Wan first appear in the movie.