As i said in an other post, i'm a firm supporter of a revamp of OoG, which doesn't work great to me, into something giving more space to units coming from the evil men of Rhudaur, and giving more of a sense of an active, quite logical, counter from Sauron to the fictionnal reemergence of Eriador and later Arnor under the command of Aragorn, instead than just a bunch of flea-bitten half-savage orcs from the mountains.
To make space in terms of use for these new units we could push some of the already existing units from OoG to OotMM which suffer as well from a roster not in line with the more recent ones both in term of interest and diversity, which would bring some welcome complexity to this faction as well. As a very personnal suggestion, i would be interested in seeing the Gundabad faction changed both visualy and in terms of gameplay by making them more of a balanced mix of evil men and orcs, more in the 50/50 line than the 85/15 ratio like Isengard, and eventually with new gameplay elements coming from the inherent difficulty for a general to raise and maintain troops so differents in such high number. This way they wouldn't be "just another orc faction".
However i'm not a huge fan of the idea of a "reemergence" objective for them like for Arnor, simply because, it looks unlikely to me than someday Sauron goes see the With-King and says something like "Hey, mate, remember when you used to control Angmar and stuff? Ruining the life of those pesky humans? Haha, mate, that sure was good time! It would be good if you could go back to Carn Dum and try to make it all cool like it was, again". Sauron would not give a damn imho to recreate Angamr the way it was, he wouldn't see any glory or self contained objective in the reemergence of a powerful evil kingdom up North, like it is for Arnor in the eyes of Aragorn, he would just see them as a tool, a powerful one, a scary one, to break Eriador before they become too much of a problem. There is a strong difference between Sauron willing to see a powerful force in the North, and willing to see Angmar back. And i don't think he's a very nostalgic dude.