Well, for the ground monsters, you can make some them fire at the player, but they will definitely make the world more "alive", I think.
For jumping monsters, you could do monsters which can only jump too (they will jump left or right, depending on the player position), they are quite funny !
Boss fights would be great ! What a shooter without boss ?
Oh ! Ships and submarines in water worlds would be nice !
Cirrus Minor: no link!? (P.S. 'love your astronomy name) I'd be interested to see your game! (I'll steal cool ideas from your game if you'll steal cool ideas from mine haha)
Oh, link to my game are in my signature (and I've stolen most of the idea of Binding of Isaac and the classic rogue games
)