Need help balancing homebrew wizard subclass for player

Hi everyone. Thanks for reading my post. I'm a relatively new DM (only 15 years experience) and one of my players has asked me to help design and approve a custom subclass for him. He called it the "Grand Wizard." I'm having trouble figuring out how to balance it, and I'd appreciate some advice.

The player, "David," is great. very enthusiastic, always shows up on time, never causes table trouble. He has a clear vision for this character and he's been specific about what he wants. He says the Grand Wizard is a mighty spellcaster who draws power from ancestral heritage and the purity of tradition. He wants the subclass to have abilities that focus on preserving the heritage of his people. I think that's a cool thematic angle, kind of like a druid but more focused on lineage than nature.

He sent me a list of features he'd like the subclass to include, like a special robe that gives them bonuses to saving throws. He also wants a hood that makes the character look mysterious and imposing, for advantage on Intimidation checks. He asked for a spell called p**urifying torch that only targets creatures of "impure heritage." I told him I'd need him to define what that means mechanically and he rattled off a list from memory, I don't remember them all, but he definitely mentioned orcs and tieflings and drow, as well as a lot of other playable species. I said that sounds like a narrow focus, and he didn't disagree, and the conversation kind of trailed off from there.

I feel like I can't say no to this, because he's so excited. He has said this subclass idea is the culmination of a lifetime of dreaming and that he's been waiting for the right campaign to play this character and the right DM to allow it. He also mentioned that he has several friends who might want to join and play as the same subclass so they could form a whole clan of Grand Wizards.

Has anyone else designed a subclass like this? How do I balance the "purity" mechanics without making it too niche or too powerful? Also, is there a lore reason why this subclass would be called "Grand Wizard"? I looked around and couldn't find anything in the official books. Maybe it's from a third-party supplement?

Thanks for reading. If your advice is "this seems like a bad idea," please explain why because I don't see it. I think it's a cool and unique concept, and I want to make it work for David.