As someone who loves Bretons and Argonians, I would love to see both High Rock and Black Marsh.
However, I think Black Marsh more-so, due in parts to it being different, to say the least.
I want to see the different architecture. The different culture. The difference weapons, and the Argonian-designed armour - you know, armour that is design around their horns, frills, and tails. It really bugs me in TES games when the horns disapear, the face/snout gets squished, and the tail just seems to go magically go through everything. Clearly these pieces of clothing/armour were not designed for Argoanias, and practicaly, they shouldn't even fit (I know it's a game mechanic, but that's besides the point - it's immerision-breaking).
Argonian culture is a huge selling point for me, and so is were-crocs!