I was hoping for a switch to Microsoft's .Net, which would enable use of Arcadia (Clojure in Unity3D) for serious/big tasks.

Right now, the GC performance of Unity is horrible relative to what you'd get on the JVM and there's no reason to believe IL2CPP is going to be any better.

When you don't have a generational collector, you have to seriously constrain your allocations, crimping your immutable style.

The Unity roadmap suggests they're going ahead with a Mono upgrade (I believe they dynamically link every platform sans iOS):

http://unity3d.com/unity/roadmap/

So it's probably not on the cards (no pun intended). However, the Mono upgrade will bring a generational GC to the platforms on which it's usable (which will fall somewhere on the spectrum of Editor, PC, !iOS).