For me as long as the series is rather unique I dont mind much since if that series end there will be nothing more like it.
For example: I would like to see a Timesplitters sequel, however...
Zhukov said:
More games in series to tell an ongoing story? Sure.
Sticking another game onto a completed successful one because you know it will sell? Eh, I'd prefer something new.
...Zhukov is right, and to be honest the story of Timesplitters doesnt really have anywhere else to go so for that I guess the best thing to do would be like what they did with Goldeneye/Perfect Dark and Demon Souls/Dark Souls where its basicly the same game mechanics being applied to a new IP.
Although the time travell nature of the story of Timesplitters is rather important for the game itself since its the reason why you can play a deathmatch game against a robot weilding a crossbow in a 70's disco using a cowboy with a flamethrower.