With the new season starting soon, here's the list of characters that should get moar development/episodes.
Finn: In a dramatic turn of events
Jake: We really don't see much of his past or anything
Flame Princess: DUH? She needs moar episodes
Peppermint Butler: He's so creepers
Lady Rainicorn: Is pregnant
Ice King: See Marceline
Marceline: See Ice King
LSP: She was in like, 2 episodes in season 4. UNACCEPTABLE
Beemo: We love Beemo. You better say you do ಠ_ಠ
Hunson Abadeer: Part of Marceline's past
Lich: Needs a personality
Prismo: Needs a girlfriend
Aaaaaaaaaaaaaaaaand Billy: Because I said so