No, I don't mean an XBLA release of Marathon, as it seems that is not to be; but rather, what was thought to be a long-dormant project to make a total conversion of Marathon 1 for the Unreal Tournament engine-- Marathon Resurrection-- has finally been released as a Final Candidate.

If you find the open source version of Marathon, Aleph One, still too low-tech for your tastes, but still want to sample Bungie's pre-Halo SF trilogy, have a look.