So far he's the only Shrek villain confirmed to still be around (though I think Charming is also since its hinted the tower that fell on him in 3 was just a prop, however it hasn't been officially confirmed anywhere) and he was imprisoned at Shreks swamp at the end of 4. I'm also pretty sure it was revealed he was still trapped there in either the Christmas or Halloween special .
My theory is that its going to be revealed he escaped capture at Shrek's swamp sometime before the events of Shrek 5 and will act as a henchmen to the 5th bad guy to have his revenge on Shrek. He's most definitely going to be in it since Walt Dohrn is directing im just not sure what role rumpel is going to play. My biggest worry is he gets a redemption arc which would undo everything that made him a great villain, but its unlikely with how evil he was in Shrek 4