Is it possible to replace JS with Lua, or is that a little more difficult than what is in mind?
I lied. It's not Mono compatible! Welp. I was running and testing code under MonoDevelop (now titled Xamarin Studio) but it seems it was using .NET 4.0? When I run it exclusively under mono it doesn't seem to work. Anyways, JSIL may not be mono-compat as well, I juts hope it's cross-platform in the end (I mean it's JavaScript in the end)
There are already a lot of LUA engines, it's a very popular language for scripting games. But what makes Sphere interesting is that it uses JavaScript, and has a lot of freedom (most other JS-scripted engines I've found really don't). In fact, what I would consider the most similar engine in spirit (or how I see the spirit, anyway) to Sphere, Love2D, uses LUA.
...Well, I'm saying that even though I like JS and know a bunch of details in JS I don't know how to do in any other language. So JS is optimal for me, and sticking with it just strengthens my knowledge on it. Though that makes me a one-sided coder.