Well they COULD end up acting like Shiva and just using a person as a vessel, but indeed it doesn't make much sense because, as Kyle said, they could've used Gaius and the person seems to keep be able to command the new power. But we're talking about a primal and a 'god' so idk, maybe the god would get complete control of the vessel.

Maybe they're just making us stronger so Hydaelyn is forced to spend energ fiing the planet AND helping us.