A spin-off could work in the sense of being in the same universe but still, for the most part, unrelated. An example could be that Foster's was able to expand and, thus, we watch the exploits of a new home for imaginary friends.
Maybe instead of Mac, we have a little girl who could find herself in a similar situation with her own imaginary friend (or lack thereof). Familiar faces could show up once in a while too, like Frankie and Mr. Harriman showing up to make sure that everything in this new home is alright and running smoothly. Anything is possible really.
