Hmm, conjecture.
Which do you fellows think will hold more water: the human appearance of 'akaviri' ghosts in oblivion, or the contradicting claims calling them naga or serpents?
True, the ghosts' appearance was probably to skimp on making another bonafide race, but if they are indeed naga as the (dubious) history texts claim, wouldn't it have made more sense to use an argonian rather than an imperial?
I think Bethesda admitted they didn't want to make new models just for a few ghosts. Doesn't mean those ghosts are tsaesci though, could be the descendants of nedic people who went to Akavir for some reason (remember that the islands between Tamriel and Akavir are inhabited, it's not impossible that some nede went further east) that were assimilated into the tsaesci empire.
Unrelated question: where did the beastfolk come from?
Khajiiti are bosmer changed by Azura into cat people (the other account is them being aboriginals, but that's dull). Argonians, Sload, Imga and (extinct) Lilmothiit have always been around, at least since the Kalpa started.
NINJAEDIT:
Argonians are possibly related to giants. Or possibly the Hist are related to giants. They share a common ancestor or something, ancestor spirits distinct from the Mer. I don't know too much about it.
Nords claim in their myths (
one of the fights of the aldudagga, actually) that they and the giants are related. While argonians and hist have a lot of things between each other, they have nothing to do with nords or giants. Based on the two novels, it seems that the hist have something to do with Oblivion, Clavicus Vile in special.