I would actually casualize the shit out of it. Streamline combat, make it so that most minor combat encounters end after like 5 turns max. Collapse skills down into like 10 max, maybe even combine two attributes to lower that number. Remove all the component stuff from spells, you can cast a spell as long as you aren't unconscious. I think the D20 system is too ingrained to change now, but I'd want to try something that's faster to grok, either by going for a system where you roll under your stat or moving to more D6s. I'd also want to jazz up character building.

Basically, make D&D into actually babby's first tabletop RPG. It's currently treated as such, despite being a terrible system for newcomers IMO. Most of the new crowd of TTRPG players I've played with want to hangout with friends, do some silly improv bits, maybe have a cool moment where they kill a goblin in a creative way, and go home. D&D is way, way too crunchy for that crowd, they quickly stop paying attention during combat if it drags too long, and nothing drags a game to a halt like one of those players needing to make a check they haven't "practiced" before. Assuming I'm still trying to do Cynthia's job of making more money than God, I think that's the clear way forward, because all the videogames and movies and marketing don't mean a damn thing if the new people bounce off the ruleset.