@YourPalWaver
I don’t have any solid guidelines to offer. I usually just rank the submissions based on what I feel would look best, and approve as many as necessary until I have enough. Generally though, what I look for:
Art style - clean, smoothly lined, not too overly detailed, show accurate proportions
OC Design - easy and pleasant on the eyes, color and design doesn’t stand out heavily, believable canon-esque appearance
Basically, I try to arrange the entire collab so that no one character stands out too much (Unless otherwise desired). Since flying OCs are much more visible, I want them to not be overly stand-out-ish beyond that. The more the OC/art style draws attention to itself, the less likely it’ll be to get a yes.
That’s not to say that the criteria is overly harsh. It’s all about balance. You can also look at the previous collabs to get an idea of what I accept.