What would the ideal Altmer look like to say, a fourth era Thalmor? Would their skin be darker or lighter? Would their hair be lighter or darker (golden, white, dark, etc.)? What about eye color? I'm looking for the appearance that would best highlight some of the genetics many Altmer seek to preserve.

I assume light skin and light hair. After all they always go on about their "Golden" skin. For eye color? I don't know, maybe yellow?

The typical Altmer image is seen as light golden skin, gold eyes, and white or blonde hair. However, I'm fairly certain that a pure-bred Altmer from the Isles can be diverse with darker gold complexions, eye colors that aren't purely gold, and hair colors such as brown, auburn, gray, and even darker colors (which I ask about regarding pure-blooded Altmer in a topic of mine here).

But since you ask for the "Ideal Altmer" appearance, it's golden skin (yet paler than a Bosmer's tan complexion), gold eyes, and white or blonde hair that's normally thought of when one hears about an Altmer.

Arcano is considered good looking based on comments made. I would go with a similar look