Enemies should keep the game fresh with new visuals and mechanics.
New areas become interesting if player doesn't know what is coming next, adds novelty to the game.
Exclusivity of some enemies to specific areas is good.
Distinct silhouettes.
Creating enemies by reskinning / palette swaps make it easier to produce a vast cast of enemies yet it causes the game to become boring.
Enemies should share art style and tone.
Enemies should feel different and present the player with problems that can be solved in different ways.