I would love to expand on the point once I'm back in front of my computer, but nationalism - if it develops in any recognizable form at all - is unlikely to make headway in the Roman sphere. It's a pan-national empire with a dominant religion and preferred language, but it’s been pan national for a long time now, and its emperors from numerous backgrounds (a tradition they can trace all the way back to the 1st century) certainly reinforce that image. It feels like nationalism is going to be a very peripheral thing ITTL, and I'd go so far as to say that it'll become a bit of a chicken or egg question about the buffer and border states/tribes.