Mario Party-esque party gamemode (client mods are justifiable)
A co-operative dungeon crawling gamemode or rogue-like
Sonic & All-Stars Racing Transformed-like gamemode (likely possible to execute decently with some clever coding)
then one that wouldn't be code-heavy:
Platformer challenge using some interesting mechanics (maybe
mine??? *rapid eyebrow raising* heheheoheuhe i'm such an egotistical jerkhole i'm ashamed of myself)