What would he looked like?
Would he looked like in LoD cinematic where he sits on a throne carried by his minions?
Or in his final battle spider form?
Or would he looked like Tal Rasha in Diablo 2 cinematic act 4 Enter Hell?
Personally I would like him to start the match in weak Tal Rasha form and after reaching level 10 you press his trait and he evolved becoming more powerful and change into his LoD cinematic form sitting on a throne then after reaching level 20 you press his trait button again making him even more powerful and changed into his final battle form.
Edit: If all the 3 prime evils are on the same match, on the same team, their skills will have special effects on them. Can also be a good special event quest for hots.