Odin was battling Surtur everyday in the afterlife as a check and balance to Surt's chaos.
If Odin's back - where is Surtur?
Who's minding the big red demon with the heart of flame?
Surtur is probably back in Muspelheim plotting his next attack on Asgard. It's the kind of thing that might not come to the fore for awhile, but eventually somebody will want to use Surtur again, and it should be pretty easy to pick up on the idea that, with Odin having returned, Surtur was free to return as well.