Seriously, I think the one that can't be mentioned is Babylon 5. Here's my reasoning:
1. Someone definitely HAS snatched up the license to do a B5 RPG. Because of the nature of Warner Brother's licenses, a signifficant amount of money had to be invested in order to acquire the license. There are few RPG companies that can front that kind of cash, but I would be willing to bet that Mongoose has enough to do it. Others would be WotC (unlikely), White Wolf, and possibly AEG.
2. Babylon 5 has a huge following in the UK.
3. On a separate thread about Babylon 5, Matt took the time to comment. When asked if Mongoose had the license or if they could comment on it, Matt simply answered "No." Is it no, they don't, or no the can't comment? There was no further explanation, but usually when a publisher does not have a license, or is not doing a product of a particular type, they usually make it known in no uncertain terms when asked.
4. Quote by Rhomus: "Whatever the licence is, it's something Mongoose feel needs 3D artists, so it's probably something that heavily features vehicles." - Babylon 5 fits that bill precisely.
I'm about 85% certain that it's Babylon 5. I would be willing to put money on it. Any takers?