>To me it could discourage people from experimenting new roles
If there's no domain, you can just go ahead and do pretty much anything! (You can do anything in service of your roles, and as individual action you can do anything in service of the organisation provided it creates fewer tensions than it solves).
Role allocation is the domain that the LL has by default. You can request to impact that domain, and the LL can technically refuse only if they have a valid objection (like "I don't believe you can process the tensions in this role effectively").
As ever, I think of governance records as doing two things: (1) keeping an accurate enough model of what's happening in the business (2) framing the conversations that need to happen. So if you are effectively doing a role, it probably helps the LL (and the whole organisation) for you to be allocated to that role - then the model is made more accurate. If the LL doesn't want to add you, sounds like a tension and conversion that needs to happen is being set up!