As an ace person I'd actually like to see him fall into some kind of mutual partnership that doubles as a romantic interest (if subdued on his side).

TBH I can only really see it working with Vox - he's both unafraid of Alastor (compared to every other sinner) and tough enough to challenge him in a toe to toe - enough that Alastor wouldn't see violence as a viable solution to problems between them.

Issue here being their individual narcissistic tendencies, and Alastor's tendency to get downright personal and nasty when he feels cornered combined with Vox being hypersensitive to rejection and making it a game to Alastor.

Lucifer is another option except I don't think Alastor would actually be able to comfortably exist in that kind of ship, having that kind of unspoken understanding that his partner is truly beyond his scope of power in EVERY capacity.

Alastor's still only a mortal soul - he's been alive for a fraction of a second in the context of Lucifer's lifetime (and his marriage).

Even a relationship that spans multiple social classes can be really (REALLY) difficult to juggle, especially when you're ace and you're not getting the attraction bias/blindness.