I'd love to see some of the old engines come back to the game. And some new official suppliers rather than just putting their badge on another's. I also think engines suppliers from NASCAR and IndyCar could make a decent attempt. While the specs are totally different, those engines used in the endurance races like LeMans are fast, and they can last close to 5000km in one race. The whole F1 season is around 6500km.
Alfa Romeo
Aston Martin
Chevrolet
Ford
Nissan
Porsche
Toyota
I think it would also be nice to have one more US-based team (and probably use the Ford or Chevy engine), and one from Japan that can be a sort of home base for Honda engines. Lastly, I think Dallara can make a car of their own since they are a major chassis supplier, both to Haas F1 Team and IndyCar.