Other thanthat, I still say we should get another epic from Brad Bird - The Incredibles was great, and Ratatouille was inspiring, and unlike most other Pixar movies, he doesn't rely on necessarily A-list celebrities who have done very little voice work to provide the voices for the characters (exception being maybe Samuel L. Jackson as Frozone).
Brad Bird is supposedly working on a live action movie that Pixar is co-Producing at the moment. Rumor has it, he's mulling over an Incredibles sequel, and will do one if he comes up with a story he (and the rest of Pixar) feel worthy enough.
As for Bug's Life... I wonder how a 2 would work. I certainly don't want any Grasshopper Revenge plots. Revenge plots are what I hate about sequels... it ruined TMNT II. That's why Tim Burton and the writers of the Batman movies killed off the bad guys at the end of the pictures. A spinoff movie about P.T. Flea's Circus would work, though... there's a lot of potential there.
There are several Pixar movies I can't see sequels to... and Spoiler if you didn't see any of them, though I doubt anyone missed any by now.
Ratatouille... Remy gained secret acceptance as a chef, his dream has been fulfilled. Nothing more to see there. Plus, I don't think they'd find that a financially good idea. it didn't bomb by any means, but that was a pretty adult movie they made. Not sexually adult... just themes that are too sophisticated for most kids. heck, most adults. It's a LOT of food and wine snobbery and in jokes.
Up... they went to Paradise Falls... what else can you do? Keep on Exploring? That would be repetitive, and it would work fine for a series of comic books or something, just not as a movie.
Wall*E... the humans are back on Planet Earth and presumably learned their lessons about dangerous levels of consumerism and pollution, Wall*E and Eve hooked up... not really much else to it, unless they did a prequel too.