The Custom Powerups patch is a package of a bunch of ASM hacks mainly created by MarioE and LX5 with the main goal of adding new powerups for Super Mario World with ease and give a bunch of tools for programmers to create their own powerups for the game.

• Several new powerups from various Mario (and other) games
• 32x32 player tilemap
• Dynamic powerup items and projectiles
• Easy and powerful customization options in the main patch and each powerup
• Enhanced vanilla blocks and powerup items to make them more interesting
• Better Powerdown patch native integration

• A nice ecosystem with RAM defines ready to use in every major tool (PIXI, GPS and UberASM Tool)
• Frees up ~22000 bytes of RAM at $7E2000
• Can insert up to 256 different powerups
• Several RAM addresses to alter the behavior of the patch's core engines
• Easy to use "add-on" engine to include specific ASM hacks for your powerups
• Total control of player's interaction field with ground and sprites
• A bunch of pointers to routines and tables ready to use at $02800C about various things of the patch
• Let's you install external ASM hacks that need to run during NMI at $00A304

Follow the guide over here