I'd be lying if I said I thought a b c.... But I do hope he is the actual bad being referred to, and not Thanos as some others have said.
Hell, I wouldn't mind if Avengers 3 involved the avengers needing
Thanos' help in defeating Surtur!
And Avengers 4 would be them learning that letting Surtur rampage for a while might have been a better idea than asking for Thanos' help
All that fanfic desire aside, I just want Surtur to the big hidden bad in Thor TDW.