Timesplitters 4, but according to the series developer, the game still didn't sell as well as hoped, which is why they never worked on a new game.
According to Free Radical lead artist Tristan Reidford in an interview with Play Magazine, "TimeSplitters: Future Perfect didn't perform as well as expected in the shops. Although fun to make, without stellar sales figures a further TimeSplitters game was going to be a tough one to sell to publishers. It's a hard game to summarise in an easy sentence like 'car modding racing game' or 'third-person stealth game'."
The fact that Future Perfect bombed in the box office wasn't the sole reason a Timesplitters 4 didn't materialize. The developers also wanted to try something new. "I think we just felt that the franchise was well-received but we had other things we wanted to try before going back to it," stated lead animator James Cunliffe. "We weren't in any particular hurry to rush into TimeSplitters 4."
Free Radical's post TimeSplitters experiences were disastrous, however. The PlayStation 3 shooter Haze was critically panned and a commercial failure, while their Star Wars Battlefront III project was axed by LucasArts.
The developer was bought by Crytek, renamed Crytek UK and worked on the multiplayer for Crysis 2. Crytek owns the rights to Timesplitters now, so it'll be up to them on whether a new Timesplitters sees the light of day.