As a collaborator, I think that open and honest communication is paramount. Everyone should feel comfortable asking questions, pushing back or compromising (to some degree!), and having a good idea of expectations, timelines, and what the collab process will be like.
I’d also say freedom to create is important, and I’d expect a partner to push back at times but not necessarily to stifle creativity but just to reign it in / help it make sense (someone who may collaborate may have little working knowledge of the development process or the real demographic of the brand, whereas the brand will have more experience, so they can offer some guidance that is useful!).
Lastly, commitment to quality – and that means product quality but also quality of customer service and problem resolution once the collaboration releases. As a collaborator, a lot of that can be out of their hands/control so that’s something that has to be honored by the brand.
As a consumer of collaboration products, I want to see something that reads authentic to the partner, both in terms of whatever colors, types of products, names, etc. that used. Brands should be willing to restock or otherwise have excessive quantities so that it doesn’t sell out so quickly that it leaves a lot of people out of luck.