I think the ability to make a custom Terms of Service, and be able to make different ones for different listings, would be extremely helpful for artists that take larger projects, or projects that have a legal copyright aspect to it (ie graphic design / logo design). And obviously there should be some sort of hiearchy as to the Commissio ToS or Artist ToS, if there are conflicts between the two. 

Licenses exists and are a fantastic feature, but do not give artists a lot of control over it. 

Creator Guidelines exist, but apply to every single project and don't have any clear agreement function, other than writing "By commissioning me you agree to this."

Things I would love to see specifically:

1. A ToS section on the Project Creation page for the customer
2. Options for setting agreement to be required to start the project or not. Without requirement, the customer can still start the project without agreeing to it, to allow communication and discussion first. And, with the Project Phases suggestion, an ability to request agreement at a certain phase before going forward. 
3. Custom ToS section for individual listings, or content sections if those are added. Possibly the ability to write up multiple, and choose from them when making a listing.