yes at this point I think the Argent Lordaeronians should be just Horde at this point, since the Forsaken are literally their own brethren and it would be more sensible to renew their alliance with the Blood Elves, as they were allies before and protecting them from the Alliance would be their sincere reparation after Garithos attempted to mass murder Kael/Rommath and their comrades given that the Argents are the closest descendants of Garithos' New Alliance Remnants. Also Eligor Dawnbringer and Korfax are friends with Eitrigg and Maxwell Tyrosus is implied to be acquantances with Liadrin.
I think it can be done lorewise by making Turalyon sanction them for doing nothing in the Fourth War and on Sylvanas' actions in Southern Lordaeron, and pulling out the Alliance from all of Eastweald and cutting off trade with them and then the Argents can only seek help with the Horde for protection
But still I prefer Alteraci Humans to be playable and the Lordaeronians to be just special non-playable allies like the Highvale/Silver Covenant on the Alliance
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I don't know why Quel'thalas proper would ever want to be with the Alliance; the Blood Elves will never forget the Alliance's neglect for them for 6 years and they all would've died there were if not for Rommath's miracle and the Horde's assistance