Just looking at Urban Arcana, which is somehow 17 years old. That right there, 100%, should be a priority project for WotC, as a D&D campaign sourcebook.
And as a D&D sourcebook, it should be 100% compatible with the existing system. My current thinking is that real world "mundanes" could just be modeled along the lines of the Expert class from Tasha's. From either the class or some new backgrounds, they get general knowledge of the modern world, proficiency in modern land vehicles and computers, etc.
Once they get into the crossover fantasy stuff, they can multiclass into any standard D&D class, or continue to progress as "Moderns" or whatever you want to call that class. Heroes who make the opposite journey --- Faeruninian adventurers stuck in modern NYC, for example --- might instead take a level in Modern as they adjust to the modern tech.
There are already some Unearthed Arcana spells for techno-wizards. Plenty of design room for similar ideas --- urban druids, Internet bards, and probably a number of classes specifically oriented towards the phenomenon of crossovers.
The fluff from Urban Arcana is excellent stuff, and could be used almost as-is, but alternate campaigns from Harry Potter to Percy Jackson to 80s cartoon D&D are all possible.
I'd love a 2nd edition of d20 Modern, I really liked having a D&D compatible modern-day RPG. Urban Arcana was a pretty good expansion for it.
The things in d20 Modern that people never understood or worked well, in my opinion:
1. The base/advanced class system. People hated having to start as a Strong Hero or a Dedicated Hero or whatever and that they couldn't have a character class like "Soldier" or "Medic" until at least 4th level. The intent was clearly to make a modular, adaptable class system that could depict virtually any modern character, but players like having their character concept and class line up from 1st level.
2. The wealth system. I understood it, but MANY people didn't. On the old WotC message boards, I swear at least a quarter of the threads there on the d20 Modern board were people asking about or complaining about the Wealth system because they (or their GM) didn't understand it. It had a nice concept, to be able to reflect modern finance and credit, but it made things more technical than a typical player was willing to learn.
3. Not using Wound/Vitality. I know this was done intentionally to make it more compatible with D&D for crossovers, but the WP/VP system would work better for a modern-day game, where a single gunshot could potentially take down even a 20th level character.
I didn't care much for the Urban Arcana setting though. It was trying to be a crossover setting from the real world to D&D, but I didn't like a lot of elements of the setting.
1. Trying to say the Plane of Shadow is the ONLY plane accessible from Earth. The Astral Plane (and to a lesser extent the Ethereal Plane) are real-world occult/paranormal concepts. Trying to say they don't exist on D&D Earth seems genuinely silly and contrived.
2. Trying to say there are lots of D&D fantasy creatures like kobolds, ogres and mind flayers running around, except nobody except other supernatural creatures can see what they "really" are and just assumes they're funny looking humans.
I think a world that has become more used to modern fantasy works, like Harry Potter, could do a D&D-in-the-real-world setting that handled the themes better than Urban Arcana did.
