Vikings fought pretty much everywhere.
In Great Britain you could have Late Romans, Saxons, Welsh, and then a bit later add Normans, and then on the advance into France you could have Franks and Germanic tribes.
I read through the book over the Bash weekend and I am going to do a Merry Men and Sheriff's match up - I had forgotten all the suggested warbands in the back of the book.
Saracens and Crusaders is another ................... and they all could easily be used for SAGA / Crescent & The Cross games as well.