I agree that the Anglo-Saxon animal interlace is far too evocative of Rohan, the Celtic swirls and knotwork aren't right for the Dunedain, and frankly, I think the Art Nouveau seems just a little out of place in Middle-earth, elves aside. Where that leaves the Dunedain, I'm not sure. Like I said, I'm generally not very helpful.Trying to piece them together to get third age Dunedain design is like trying to reconstruct 17th century German Jewish motifs from a Sumerian bas-relief, a couple clay tablets, and some 12th century Sufi calligraphy.
My basic "feel" on the Dunedain is one of a "hopeful sorrow" if that make sense. They are the remnants of a broken society and yet they are holding out for Arnor to be reclaimed. Determination, solitude and sorrow seem to be the main traits - to me at least. Especially with the Rangers being the self-imposed outcasts tasked with protecting a society that fears and doesn't understand them. How one would express that through art is beyond my knowledge and I'm not even sure that it would be expressed in their art. I know the Professor seems to prefer a sort of static society but I find it somewhat of a stretch that the Dunedain would retain Númenórean motifs throughout the millennia and all the fire and change that brings. It's a puzzle that I'm not qualified to solve honestly, ramblings aside.
I also agree that "no other fair thing" doesn't mean plain and I definitely think there should be some sort of cultural decoration, whatever it may be.