Barbarians are very solid characters in these games.
I finished the first game with a Barbarian who did take a few Fighter abilities (mostly Veteran's Recovery) so it made sense to make him a Brute (Fighter Unbroken/Barbarian) in Deadfire.
Frankly I believe that any combination can work and it seems to me that multiclass allows for some versatility without being over the top while going pure class makes it more focused (which is as it should be).
My advice would be to look at the talent trees and see if that fits the concept you have (there are also kits for every classes but you don't have to take one).
Some people have reported issues when exporting their save from the first game to the sequel but as far as I'm concerned I didn't run into any problem (but I had just finished the game so it was with the latest version of PoE).
Considering the Watcher thing maybe taking a Cypher class would make sense but I'm still on the fence when it comes to these guys (just a personal bias, not saying they can't rock).
"Growing old is inevitable but growing up is optional."