Personally I don't think 5E should have a Monster Manual in the classic sense.
I'd prefer to see a boxed set approach where Levels 1-5, 6-10, 11-20, 21-30 and 31-50 have their own DM rules portion, player portion and monster portion.
However, I think the key to an iconic cover is in illustrating an encounter in progress. So it should feature a PC (or PCs) actually doing something (not posing for the 'camera') and a location. The scene may also feature monsters fighting the PCs (or it might just be a trapped room or wondrous location).
Assuming the core rulebooks are released in the same fashion as 4E (which I don't advocate)
- Players Handbook: Group of PCs having just slain a dragon.
- Dungeon Master's Guide: A Dragon having just slain a bunch of PCs (or certainly in the process of slaying them)
- Monster Manual: Group of PCs battling against Demodragon.
However I do prefer the boxed set approach.
- Red Box: Large dragon attacking a mounted Knight and four other iconic PCs.
- Green Box: Huge lightning-spitting dracolich attacking the five iconic PCs. One of the five PCs is trapped within its ribcage.
- Blue Box: Demodragon attacking a castle, while the iconic PCs try to repel it.
- Black Box: Tiamat (about 300 feet tall) being attacked by the five iconic PCs who are flying around her (like a dogfight) all mounted on gold dragons.
- Gold Box: Draeden (with dragon-headed tentacles) attacking a planet while the iconic PCs fight the monster in space.