The factory swap I would’ve loved to see is the HO Quad 4 and Getrag 5-speed w/torsen final drive that Olds used in their '88 442 put into Pontiac’s Fiero Formula or GT. That would’ve been a motor actually suited to the redesigned suspension and would’ve turned it into a real sports car.
My dream swap is the one I started nearly 40 years ago, but never completed: Pontiac 455 backed up by a TH425 from a Toronado, sitting in the back of a '66 GTO Sport Coupe. I had the motor and transaxle installed and the rear suspension fabbed, (basically grafted in a chunk of the Toro’s frame) but hadn’t built a firewall, set up the cooling system, or decided what to do with the front suspension. (Rodders were just barely starting to swap the Mustang II into stuff and I hadn’t seen any of them yet.) Then life got in the way. The front clip got stripped off and traded to a buddy and the motor got sold to some guy for his 442 and the rest went to the scrappers. I think the decklid got saved, too, but nobody was interested in the (straight, rust-free) body because it wasn’t a hardtop. My how times change!