A little more limited, but this one is the only one I've got which, of the format requests I'm posting, is completely free. Participants would upload a custom design code or (iirc) the main editor for this thing has a file format. Bitpack could contain a palette or theme.

it wld be helpful if you actually link resources to tools, file formats etc

Here's a tool that has a file format.
https://acpatterns.com/editor

okay, here are a few things about this. and i wanna add that this is just my personal opinion, not a final decision or anything, because im not the one who decides this.

i dont see why this wld make a good format.
sure, the online tool you linked saves and loads .acnl files and is available for free. therefore it meets the requirements for being a botb format technically.
but:
1) im still opposed to formats that only work with an online tool, what if the website goes offline (etc). standalone tools are a lot better imo
2) i experimented with it a little, and it just seems like a simple pixel tool, very similar to the kixel editor made by multi, but even more limited
3) you cld rly just host a pixel battle with this palette and resolution and get the same results, except the submitted files are png, not ac-compatible BUT
4) AC-compatibility is rly nothing too important for most people on here. im sure a majority of botbrs dont own this game

One difference between it and pixel, is that AC's editor uses it's own fancy interpolation method, so the final image isn't really pixel art, but more of a vector design.

I agree with the other points tho

fun idea to use the AC tool for a visuall battle (the same way that allgear can be used to host weird vst/software battles sometimes)

i like the IDEA of this a lot because it really is AC's version of doom wads or lunar magic levels. it exports to QR and can immediately be used within the game itself. the only thing that kills it, imo, is that you really need the game it's for to enjoy the results of it, and new horizons isn't some old game you can easily emulate. if all you do is look at the pixel art, then it undermines the whole point i think.