If Microsoft was smart, they would do their best to round up old Bizarre staff and re-create the studio for PGR5 as a 720 launch title.
PGR4 was launched at the worst possible time. It was released October 2nd 2007 1 week after Halo 3 launched on September 25th 2007. Had the release not been so close to Halo 3, I'm sure it would have done better.
I think a Playground Games feat. Lucid would probably cover most of it. The key people you'd be missing out on would be Gareth Wilson who's at Sumo and Nick Wiswell, who's now the main audio guy at T10.
I'm also not sure where Martyn Chudley ended up after the whole thing.
Going through MobyGames, some key people and where they are (if they're relevant/actually have updated entries):
PGR4:
Alan Mealor, Lead City Environment Artist. Now at Playground.
Stephen McAuley, Graphics Programmer. Only worked on Far Cry 3 since Blood Stone.
Ged Talbot, Lead Designer. Now with Evolution Studios (Motorstorm).
Jan van Valburg, Lead GFX Programmer: Now "part of the Activision Central Technology group". So I guess, working on the next CoD engine :/
PS. This is a promising retweet: