...from the TS games, what would you change?
In TimeSplitters 1, I would either recreate the Story levels to be longer or allow the bots to spawn with weapons.
In TimeSplitters 2, I would give it controls from Future Perfect.
And in TimeSplitters Future Perfect, make the same kind of difficulty setting like in TimeSplitters 2. Like you get new objects, the levels are longer, enemies are placed to different places and so on.