So, I'm thinking of developing a kinda "competitive" modpack for the Mario vs Luigi gamemode in NSMB. It would have basic switches like removing the ceiling on castle, things of that nature. How do I start and can anyone help?

The best way to start is learning how to apply ASM hacks, as MvL is very limited when it comes to hacking without ASM. I've made some tutorials on the basics along with some more advanced stuff. I'd be happy to help you more either by PM or in my NSMB hacking Discord server. Hope this helped!
-Ndymario
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Here's my MvL Hacking thread

Consider joining the NSMB DS Hacking Discord Server!

#HakingNoMore