We need more Monsters!
Things i can think of for the first few levels.
Spiderling
Giant Spider
Octolings (little Octopi)
Katken (Giant Octopus that Meowed)
Cave Bat
Big Cave Bat
Monster sprites need a Minium of 5 Animation Sequences. Idle, Move, Attack and Die. Some monsters may not jump and some may have multiple attacks.
Also we need a Swimming animation. Up, down, left and right.