Mr. Taco, are you looking for design ideas, or are you looking for technical advice?

Do you plan on having a story ? Maybe I can help you with it

2D platformer idea off the top of my head:

You play a Lizard with a Hat, you are named “Hat Lizard”. Your hat grants you the power of your long tongue with which you can attack in many directions. It delivers a ranged punch sort of attack.

Throughout the game, you collect different hats which give you different tongue powers. Eat-Hat let's you eat your enemies to gain some life (but be careful, some enemies are poisonous). Fire-Hat, Ice-Hat etc allows you to burn and freeze enemies. Shock-Hat will deliver electricity, which can propagate through water (but don't you get in the water too).

Hat Lizard's job is to find his big brother named “Shoe Lizard” who has gone missing in the Ward Robe Region. The game is Hat Lizard progressing through levels until he ultimately rescues Shoe Lizard.

This unlocks Shoe Lizard as a playable character to play through the game to rescue Hat Lizard.

Bonus points for multiplayer where one of you is Hat Lizard and the other is Shoe Lizard.

You're welcome.