Wrathion's model is a modified thalassian elf model I believe. As with Nozdormu's 'elf form', which has human range skin tones but a night elf base, this can likely be chalked up as another example of a lot more leeway being granted in devising models for unique and important lore characters rather than an potential extra customisation option for humans. As Nozdormu and several other lore characters have proven, just because they have done something for a major lore character does not guarantee it is going to happen in future (in fact, far from it).
As for Void Elf Paladins, Void Elves are defined by their connection to the void, Paladins by their devotion to the light. Alleria, the first Void Elf, cannot touch a light devotee such as Turalyon (and that was before Turalyon became lightforged) and so a Void Elf Paladin is a contradiction. But of course, that's just laying out the groundwork as to why a Void Elf Paladin isn't going to happen, the corollary is your suggestion that selecting Paladin under Void Elf actually makes you a High Elf.
Which is why it is almost certainly never going to happen. A Void Elf is a transformed thalassian elf, the entire rationale behind their transformation was to create thematic and aesthetic distance between them and the Blood Elves. As the Blood Elves have become increasingly focused on the light, basing the counterpart faction around light's opposition, void, makes perfect sense. Allowing Void Elves to have the option to be a Paladin ruins that thematic division, and attempting to argue that the Void Elf Paladin would actually be a High Elf (and represented as such) crosses previously expressed red lines regarding sharing an identical aesthetic (not to mention theme) that is currently unique to the Horde with the Alliance.
A Void Elf Paladin is probably the single least likely race-class combination suggestion that isn't in the game that will be considered. You could even make a better case for an Undead Paladin.