I fully realize that Asterion is meant to be a bottom in canon, but it would also be nice to have him be a top if the MC player is a bottom. I think you should add the option for Asterion to top, even if later itshows him as versatile and bottom inclined. That's the last I'll mention of Asterion being a top.
Alrighty with that out of the way...
Consent. I know this is a bit of a 'well no shit' comment, but considering the kinds of power MC has over Asterion, I want his willingness to engage in sex with MC to be clearly consensual. Maybe have Asterion suggest sex to MC, to make it clear it's his idea?
1) I think both MC and Asterion would flirt/tease the other with the whole 'my king' and 'master' name-calling stuff. I'd be surprised if this wasn't somehow included.
2) Options for vanilla sex, and for kinky sex options. I love both, but a lot of people aren't super into kink, so make any kinky stuff optional. (But definitely still include kink!)
3) Possible types of kink
Bondage
Leather
Dom and sub (bit obvious with the whole master or king thing, can go either way!)
I will say BDSM is a delicate topic. I absolutely don't want to involve pain in any way with Asterion and sex. It doesn't matter who that pain is directed at, I would be deeply uncomfortable with pain being involved considering Asterion's past!
4) Confident Asterion! No matter what I'd definitely like to see Asterion be confident with his sexuality.
I'd also really like the idea of MC and Asterion sharing quarters to be discussed. I feel like after they take this step, it would only make sense for the two of them to share a bedroom, and a bed... 😍
5) It would be interesting (as an option for players) if Asterion's dick could either be human or mythical. Some people like that and some don't, so I feel like it should be an option to have.
Finally... I'd really love if there could be one or two smaller sex scenes, where it's more casual nudity and just exploring each other and getting used to the idea of sex, before Asterion and MC go fully into sex and possibly kink.
Or there's a sort of foreplay period before jumping into the sex scene.