I hope it will be the same with the expansions too.
Well, elves are a more likely option, especially if you remember the first Heroes game. There weren't any mages there.
If you consider the storyline, mages are more likely, because it's hard to imagine a campaign for the Necropolis without them. And you could tell the whole story that way.
And dark elves could be released as an expansion, where they could tell the whole story about them (like in the case of mages and necromancers). This is more logical.
But if you remember the five heroes for each faction in the campaigns, then elves seem more suitable to me.
So, the score is 2 - 2
Демократия есть одурачивание народа при помощи народа ради блага народа.(Уайльд Оскар)