They’d just find another actor. Just like Slink. And for Heimlich’s little segment in World of Color they had someone else do his voice, you wouldn’t even think it’s someone else!
But it’s not much to worry about since I doubt there will be a Bugs Life 2.
I would like a Bug’s Life 2 more than any other Pixar sequel, but I wouldn’t want Heimlich to be in it. Getting a new actor for Slinky I can deal with, but I feel like Joe Ranft’s characters shouldn’t be done by anyone else, seeing as they couldn’t find anyone to replace his temp voice in the first place. It seems wrong somehow.
I think in a sequel, Molt could take Heimlich’s place in the Slim/Francis/Heimlich comedic trio. Molt was really funny in the first one, so I’d love to see his character expand.
Like I said, I don’t think another sequel would fare to well with the critics after Cars 2. An Incredibles sequel would be nice but I doubt that’ll happen for quite some time.