You could also do a bit of a twist:Yeah - a similar thought came to mind too. That would be very cool - trying to avoid demons being the big bad though
My current thought is that in the new time-line he "borrowed" the "tome of all knowledge" from Ioun and thence had it stolen. He's now been imprisoned by Ioun in Carceri and will be released only when the "tome" has been returned to its proper home. So some arch-lich is the big bad in this story - and perhaps they have the pocket dimension? - and perhaps that pocket dimension is in some magic artifact back in the original wizards study - (the arch-lich snuck out, took the book and is now plotting some big return?) Oooh - perhaps it's in some crystal ball/snowglobe type of thing?
The wizard is imprisoned via imprisonment spell, wherever you seem appropriate. The PCs work to free him (find out the exit condition and get him out). When they do, they find out the wizard actually imprisoned himself to avoid enemies (if you're a loner, the hedged prison version of the spell can be pretty nice, and you can set your own exit condition).
When they free him, that's when the bad guys try to grab him for his TRUE punishment.
Just a thought.