The problem with the former is that the art team goes from having to create mesh/texture resources for 10 stock races to creating 15 or so, especially for the Khajiit races which are rather physiologically different from each other. That runs the risk of lowering the detail of each individual race's look just to allow for more race sub-types. Which in my view is a problem.
The problem with the latter was the problem present in Oblivion: everyone used the exact same body meshes, and mer/humans with the exception of Dunmer all used the exact same textures. There was such a sense of "sameness" there, a lack of unique quality. Taking this further, if they create sub-races but use the same stock textures and meshes, that makes the sub-races dull and boring.
For me, the perfect solution is scrapping sub-races altogether, putting more time and energy into actually fleshing out the 10 stock races as they are, in both visual quality and culture. If too many options get included, those options will all become watered down versions of themselves. And to me, nothing is worse than content that is watered down.
I know that chances are they aren't gonna be included. However, they could all use the same head/body mesh and easily morph it for each race. Textures wouldn't be over-complicated, since they don't require insane amounts of time to make. For Ohmes-Raht Khajiit, they could just put on the tail mesh of the regular Khajiit and add fur to the human face texture, and for the Cathay they could simply make them larger than the other Khajiit and have a different pattern on the fur (which you can do easily if you have a texture-editing program with layers).
They wouldn't need unique animations, as all people in Oblivion use the same animations, even Imperials and Khajiit have the same running animation. If they have a new swimming animation for Argonians, they could easily apply that to all Argonians, it wouldn't be too much of a problem in that standpoint.
I couldn't imagine a race that looks like humans with fur and a tail needing any new unique meshes/animations, and same with a race that's only larger and stronger than the Khajiit race we're used to.
Naga and another race of Argonians could probably be made by making the Naga bigger and fiddling around in the Construction Set with textures. And I dunno about another race of Argonians, I can't picture them looking much different than the regular ones.
The way I see it, you don't have to really worry about this being included, if they're far along enough in production to be ready to show it, I highly doubt they're going to throw in some extra races. Maybe it could be made as a modding project for TES:V.
[Addition]
They can also change the height/weight of individual members of each race, and since FO3 has been out they can also add new meshes/voices. So really to create a Naga you'd just have to make a big Argonian with a "big voice", and maybe a few unique meshes which now you can add with ease to individual people.
[2nd Addition]
They wouldn't need to add entirely new cultures for each. The only difference between the Naga and the "regular" Argonians is where they live, how big they are, and some details that would make them look more "snake-like". Khajiit all live in Elsweyr, and in fact all of the "species" of Khajiit live together in all of the cities of Elsweyr, and lore-wise are supposed to exist everywhere else too. In http://www.imperial-library.info/interviews/dfcat1.jpg you play as a "Ohmes-Raht" if you're a Khajiit, and Argonians have been shifting in shape/morphology in nearly every game. So you wouldn't need to worry about them expanding on culture or anything like that, people still know very very little about Argonian culture, and Argonians in general. Ohmes-Raht and Cathay are still Khajiit, they still digest Skooma differently, they still talk in their unique way, and they still all come from the Deserts of Elsweyr. Naga are simply another kind of Argonian, like the change you saw from Morrowind to Oblivion, same thing with the Archein.