It would be better to have both mages and elves. Both races are very important for Heroes, and it's one of their defining features. And somehow, people have managed without a Necropolis so far. It would be great for an expansion pack, and also for the initial release. But, I think it would be better without sacrificing the other races.
P.S. I'd like to point out that without mages and elves, two storylines are lost, which those who have played Heroes 5 are looking forward to:
1) The appearance of necromancers (since they originated from mages)
2) The appearance of dark elves.
P.S. If mages are added, then maybe beastmen will appear; they could also be a new race.
Демократия есть одурачивание народа при помощи народа ради блага народа.(Уайльд Оскар)