Toy Story and Cars had "2" because the name really didn't give a mention of the plot. Monsters Inc could have been a 2 under that logic as well, but they wanted a prequel and a different plot, so they changed the name. In fact, before the project went into further development, it was called Monsters Inc 2.Well, they could just drop the "Finding" part and call it "Nemo 2" or something. People will still know what it is. Though like everyone else has said, I can't see much as far as sequel material goes...
So I guess they're going to need to change the title, but keep the "Nemo" part for marketing purposes. Something like, "Nemo: Lust for Glory."