The human factions should be pretty do-able in all cases, I think.
Dark age types for Rohan, Byzantines of some sort for Gondor perhaps, Arabs for Harad, maybe more Norman types for Arnor if that was included?
I guess one key question is scale; GW's are true 28mm, whereas many other fantasy sculpts will be at the slightly larger "heroic" scale, so whether one wanted to make an army that was compatible with using GW minis would have to be a factor.