Well my preference would be to make him look realistic, not impossibly gorgeous. A person can be good looking, but not look like they belong on a runway. (I also prefer human or orc, but that's just me...) I'd also suggest a mid-ground as far as age, late 20s into 30s or 40s, so he's not super young and inexperienced, but he's also not too much older; I think this has the potential to appeal to most players.
You didn't say what you're going to do with combat. I think the first thing you need to decide is the type of combat the person is going to use: physical versus magic, ranged versus melee. You can tie this to the personality as well; if the person is a risk-taker, or a protector-type, they might be a melee warrior; a cautious person might use a bow or magic from a distance.
Once you know the type of fighter they are, you can then have backstory to match. Maybe the person is a mage from Cyrodiil, so doesn't really get the whole Nord thing, and maybe even prefers you to be on the Imperial side in the civil war. While he might not leave your service, you could have comments change based on which side the player chooses. OR you could go all-out with this idea and have him be pro-Imperial, meaning he would leave if you pick the Stormcloaks (I don't know if this is possible in the CK).
For scientist/scholar/nerdy types, I really like when they have a particular subject they're into and geek out over that. Marcurio has elements of this with the Dwemer, but it isn't developed as much as it could be. Your guy could spout bits of lore in an excited tone when you're in Dwemer ruins. If you're doing a quest where you learn of other such scholars, like Sinderion in Blackreach, there should be comments about that. And so on with similar things.
What is the moral compass of your character? If they're into logic, perhaps they won't look at some of the more evil daedic quests from a moral standpoint, but more along the lines of "let's do it and see what happens."
If you've played through most of Skyrim, I think the key is to think about the kinds of situations the player can find themselves in and the potential for reactivity in those situations.
If you're going to have them be eligible for marriage, it would be nice to have some couple-oriented dialogue, stuff that is repeatable. I imagine that dialogue menu would look like this:
The current standard options for a spouse are:
I'd like you to do something.
Has the store made any money?
Can you make me something to eat?
I'd like to trade some things with you.
I'd like us to move somewhere else.
It's time for us to part ways [dismiss, return home].
Perhaps there can be an additional option like, "Other things," or "I'd like to talk to you," or something along those lines. Then you can have repeatable dialogues where the player can engage is some general banter. There can be some more general dialogue for non-spouses, while spouses have additional options with more romantic choices.
I emphasize repeatable because I think it's important for player roleplay. I don't need to have involved conversations every time -- I can imagine those -- but having the action and pretending like I'm talking would be a big help with that.