I think they should go with 5 characters.
The classic 4 races plus 1 other nonclassical but not too weird like a half-elf.
The 4 classical classes plus one other less core class but not too offensive and weird class like a ranger, paladin, or sorceror.
Foreground: 2 party members doing some non combat action like disarming a trap or healing a NPC.
Background: The rest of the party holding back the monster.
Something like the "Rituals" picture in the 4th edition PHB but with a dwarf or elf instead of a dragonborn. And a wizard blasting those drow.
What would you like to see on the cover?
I'd like to see a 4-person traditional adventuring party (human, dwarf, elf, halfling; of the classes fighter, rogue, cleric, wizard; not necessarily in that order). I'd like them to be discovering something, not fighting something. No action poses, but a sense of wonder as you share their awe as they, say, see a scary door in the lamplight. They should be ordinary looking people in realistic-looking armour/clothing, and they should have backpacks, lanterns, etc. A sense of mystery and foreboding, speaking directly to the exploration side of the game.
Something that gets your imagination going. What's behind that door?
How about you?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.