I love the idea of Nerevarine being a non-Dunmer, and I definitely think a Daedra Prince would as well. The whole concept of switching your ethnicity/class/gender during reincarnation is something I'm very fond of in fantasy settings, it opens up a rich vein of discourse on those subjects. The idea that Nerevarine must be a male Dunmer I find to be really offensive, just as a matter of the essentialism involved in thinking that only certain kinds of people can fill certain kinds of roles - sort of a "Jesus is white" thing. It's a recapitulation of the Dunmeri racism and xenophobia, and I say nuts to it! Let the Dunmer squirm with the forces of change at play, and long live gender-bending, race-mixing différance!
Nerevarine, s/he is a Mudcrab!
I certainly see where your coming from - I happen to be especially fond of the Argonians, above all races, and agree that it would be quite poetic for the Dunmer to be saved by a people they dismiss as being nothing more than pea-brained savages. However, the issue of motivation comes into play on the part of the Hero (Nerevarine), and believability on the part of the native Dunmer.
First, motivation - after being dumped into Morrowind and seeing how the natives treat their slaves (or even the free Argonians, for that matter), it would take an Argonian with saintly levels of empathy to really care about their troubles, or to not believe that the Dunmer
deserve whatever disaster is about to befall them.
Secondly, ask any native Dunmer how they feel about the Argonians. You'll find responses varying from mild annoyance at best to homicidal hatred at worst. This includes the Ashlanders, and just about every Dunmer you have to interact with through the course of the MQ (which is alot of Dunmer, let me tell you). Whether Nerevar reincarnates as a pretty Argonian female or not, the Dunmer wouldn't even
let her save them. The Ashlanders, realistically, would refuse to even entertain an Argonian's notions of greatness. You wouldn't even be allowed to be tested against the prophecies, and the MQ would be cut to an end, there and then.
That makes sense, right? I don't believe the Nerevarine has to be a Dunmer, but sometimes it becomes difficult to suspend my disbelief. The Dunmer are a hateful people, and wouldn't even give half of the races an opportunity to prove themselves. The only race with worse prospects than an Argonian or Khajiit would be an Imperial. Guh...