I mentioned it in one of the other threads, but IMO, the most likely things for the retro rumor (the one where it was said they were working on something everybody had been wanting them to do) are:
1. A "Halo killer" new IP FPS game.
2. Another Metroid Game
3. Kid Icarus (fan rumors for years associated with factor 5 and with Retro at different times)
4. That action RPG game they abandoned way back in the GC days.
All of these have been actively sought by subsets of fans at different times.
edit// add Zelda to the list because it has been asked for by fans, even though I think it is pretty unlikely.
1. A "Halo killer" new IP FPS game.
2. Another Metroid Game
3. Kid Icarus (fan rumors for years associated with factor 5 and with Retro at different times)
4. That action RPG game they abandoned way back in the GC days.
All of these have been actively sought by subsets of fans at different times.
edit// add Zelda to the list because it has been asked for by fans, even though I think it is pretty unlikely.