It would be great to have something like arcadia for unreal

Well, I'm very late to the party here, but I tried to use unreal.js with CLJS. You have the ability to simply call into / execute js files, so compiling CLJS to JS and then using it in Unreal is certainly possible, but in the end it just wasn't wort it. I also tried to embed a jvm into an unreal executable and host clj there, which worked too, but again, the experience was rather meh. You can use libpython-clj though to script the unreal editor. So, long story short. It's always an uphill battle against the engine and you're way better off just writing C/Unreal C++ to get stuff done. There's a ton of work to do to make something like CLJ a viable scripting language. But maybe embedding a lisp like https://janet-lang.org/ would be nice. FWIW I stick to CLJ for the backend and tools and write C for Unreal