I do think Calia needs a reason to get a little distance from the Alliance personally. She doesn’t have to be outwardly antagonistic to them, but if she is to be trusted to guard her now dead-people as A dead-leader, she needs to be able to make hard choices on that topic. In favor of the Forsaken, and by extension, the Horde they are a part of. I also think she should be very wary of living Human resettlement, they are likely to naturally push the Forsaken out over time. Even BtS shows that the majority of humans do not like what and who the Forsaken are for many, MANY reasons.
Beyond that though, in a scenario that includes Calia as a significant Forsaken leader … I do believe you need to build up Voss to be her counterbalance and equal. Not a temp, not lesser. Thematically, Calia represents who the Undead of Lordaeron where in life; Voss represents fairly well who they have become in death. Both are sides of their identities that should be fostered and supported. Sylvanas only really fostered the latter identity, Calia (in her current state) only fosters the prior. Voss AND Calia however could facilitate all that is “Forsaken of Lordaeron”.
EDIT: Surround Voss with supporting characters like: Belmont, Tattersail, Faranell, and Velonara; and Calia with: Faol, Derek, Leonid, and Delaryn … and you have the makings for a very solid and balanced character roster. 2 A Rankers, 8 B Rankers. A lot of variety in personality too.