Notes:

It would go something like this: Mac would grow older and have to go to college and put Bloo up for adoption. However, Bloo gets adopted immediately and doesn't get a chance to have a final goodbye to Mac. The people who adopt him turn out to run a sweatshop that uses imaginary friends as slave labor. The rest of the Foster's gang finds out about this and has to save Bloo before Mac has to leave. Somewhere along the line they find a perfectly good kid who'll adopt Bloo and he and and Mac have one last goodbye at Foster's while they drive off on opposite roads.