I prefer the native factions in. It's one of the only mods that expands on native rather than utterly replaces it.
Though I wouldn't be against some monster race rising up with a claimant that the player can champion

if we wanted to keep the number of factions static. Naga in the desert for example, lizardmen in the forests, skaven just generally being a nuisance around the center of the map, that sort of thing. You could combine lizardmen and naga too I guess that would work

. Hoards of cheap skaven sound fun too, just you and your companions trying to hold out.
What I hope for most of all is some late game challenges, a few more really, really tough unique spawns that nobody else targets, just the player for those last end game moments or battles.
But I see no problem for most computers technically just adding a race or two more at the edges of the map, its all down to the workload and time it takes.