Well, Dumbledore could still play a significant role. Even though Harry is now a mage in a different context, Dumbledore's wisdom and guidance could be crucial. Neville might also be important, having grown into a strong wizard himself and partnering with Harry in various mage - related tasks. Additionally, there could be an antagonist, a dark mage who opposes Harry and tries to undermine his newfound powers.
Hermione could be important. She might help Harry in his journey as a mage, using her intelligence to research and understand new magical concepts that Harry discovers. Ron could also be there, providing his loyalty and friendship, perhaps even joining Harry in some of his mage - related adventures. And then there could be a new character, a powerful mage who becomes Harry's mentor and teaches him the deeper secrets of magic.