In my hack, I want to make cannon levels into...well, actual levels. They'd be short, no Star Coins or anything. However, I want to add a progress bar on the bottom screen like the rest of the stages in the game, as I don't think it looks good for a level to have a big blank area there. At least Ghost Houses have Boos appear and disappear.

Is there any way to add a functional progress path in cannon levels?

Maybe you can do that if you use the Ray and Freeze world map editor, there you can edit the level numbers, maybe if you change the number you can do that, remember descompress the overlay

Here is the NSMB World map editor

https://nsmbhd.net/thread/2871-worldmap-editor-and-all-the-information-you-ll-need/

https://nsmbhd.net/post/41240/
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The star project that shows my brilliant mind

Yeah, exactly. Basically you need to make the cannon level a "normal level". The same happens in haunted house levels if you wanna make a path in the bottom screen.