Slash returned in "Donatello Trashes Slash" where he ran into an alien race of super-intelligent beings that hooked Slash up to a machine that gave him genius level intelligence (gaining a British accent as prominent evidence).
In "Night of the Rogues," Slash (alongside Rat King, Leatherhead, Tempestra, Antrax, Scumbug, and Chrome Dome) is enlisted by Shredder to help defeat the Ninja Turtles.