Hey, I have an arena server with mods on 1.15.2, and I don't want to build every arena in the same world. is there any way to use this with mods? like a mod version or data pack. I've researched and haven't found any way to do it but actual developers might have the secrets.

in the near future, no. mod work very differently as they use multi dimension and not worlds. You can try some Bukkit + forge hacks like catserver, but we don't offer much support of them.

Would kettle work? it seems to be in the right language for me, but I don't know if it goes to 1.15.2

I dont see MV supporting forge server in the near future, sorry.