Put this way: We're still waiting for an Aleph One port.
1) It's predecessor to HALO. = Relevant to X-box
2) It's open source, as are its dependencies, as I recall. = Source available
3) It has a software render mode, IIRC. = Not as much nasty DirectX/OpenGL work
4) Has had a relatively good demand.
And so far, we've seen little to nothing, unless I've missed something...
Madmab (busy with SNES9x v3 using either the 1.51 core or the 1.53 core), BP (working on CoinOPS and said to have taken up another task, while people have little true hope), and a few other small devs (the Surreal64 team still kicks if you watch their project feed, there's a new version of Final Burn Legends on the way, NestopiaX is being worked on a bit more, plus the guy chipping at Alien vs. Predator) seem to be the only "big players" left. Even A600 seems to be gone, sadly... (He may, or may not, be working on another project. I don't know.)