If you'd prefer a found family kind of dynamic-- I am HERE for it. It's my go-to and personal preference. I do not see it as infantalizing Niffty-- though I understand the apprehension. Adults can have dynamics like this too, especially ones who still need guidance in life [or in this case, death]. Plus I love the concept of the two of them sharing mutual affection for one another, without the romantic or sexual undertones.
If you are in the camp of BESTIES-- I am absolutely here for it. I once entertained the idea that Niffty owned Alastor [with the caveat that he be her best friend forever] but this was back when Season One was still airing. Recent canon has indicated he might have a soft spot for her-- and she absolutely cares about him. Again-- a dynamic founded on mutual respect and affection, even if they never really put it into words.
If you'd like to explore a romantic option, I could definitely give it a go-- they're both adults. I'm not keen on a sexual relationship between the muses, and want to make that clear. I'm all for the expression of gray-asexual beans [as a demisexual myself] I've just got little interest in that aspect of a relationship with the two. If that's what you'd prefer, there's other fantastic Niffty writers out there who could give ya that, but that's not me. Please consider one of the previous options instead!
Finally there's what I like to call I FOUND HER IN THE TRASH. Wherein Niffty is more of a minion/pet to Alastor. Someone who amuses him, supports him when he needs it, and makes him silly little gifts. She's like his emotional support gremlin-- a presence that calms him when he's agitated and quells the static in his mind.