Say if you wanted bigger worlds or a world 9, how hard would it be to add in levels? I know EFE has an "Add File" function but I heard it was very hard to add files into a DS game? Anyway, would it be as simple as adding in the file, then writing some ASM to create an extra node on the worldmap, and make the node load that level?
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Not really, I'm pretty sure that the number of levels is hard coded, though I could be wrong. But remember you can't just go about adding files expecting them to work out of thin air.

But man, making a World 9 is like stabbing yourself in the eye very slowly.
That involves making this and writing ASM for:
A method to get to it.
Bottom screen icon and map.
Flying ? blocks, hammer bros., and Bowser Jr. (If he's there)
Making the animation for Mario to get to each point on the paths.
^ Also the paths.
The Map with textures and the such.
Making the game load the levels on the points...
etc...

To put it simply; Very if you're going full out.
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"I strikingly thought you would know how but it clearly seems you have no knowledge of hacking" - MiiU 2017

 MarioFanatic64, would it be possible to replace toad houses the same way? Or are they not conventional levels?
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hey look, I did a thing

Toad houses aren't conventional levels. Starcoins won't work correctly with them, so I wouldn't advise on that idea.
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