I agree in terms of Moon Knight working generally better in his own stories, but I don't think there's no place for him on a team or in a wider universe, or that his stories or being brought up from D-list in general necessarily have to involve a team-up to be classified as better or improving.

I mean, he's that guy who probably seems weird to other heroes in their own contexts when his cross over with theirs, or he should be. He could cross with the Punishers in this potentially, but I'm pretty sure Frank would think he's gone a little off his rocker since he's dealing with werewolf potions and shadowy cabals as much as ordinary criminals and claims to be the resurrected avatar of a god since his old days, plus he uses higher tech. He could cross over with Ghost Rider or one of the other supernaturals, but he uses tech and skills moreso than magic and the world he inhabits is generally a lot less high drama and world-rending stakes.

He can actually be kind of a neat crossroads for some other heroes if he's used right because a good Moon Knight story blends mystery, military past, pavement stomping street-level stuff and at least a good pinch of weirdness and the supernatural even if their occurrences are sometimes more grounded and not always supernatural in truth.

Not to say I want to launch him directly into the big leagues even in this RP, I don't want to forget the essence of the character I'm portraying. But there's room to build him in and build him up in interesting ways.