I've been adapting Kobold Press's Zobeck to Roll20. It's taken a long time, but I have artwork, battlemaps, NPCs, background info, statblocks and more (like the laws of Zobeck) embedded in the map for ease of reference. It's pretty cool, but I would gladly have paid money for a company to release a Roll20 (or your VTT of choice) city sourcebook with all of that stuff already done and ready to click and open. Basically an interactive city where you can zoom in to street level and when needed, open up dozens of individual maps. Because it is all virtual, I have maps for everything, even non-combat.
And while they're at it, they can add moving water, birds, city ambiance, and day/night cycles.
Waterdeep or Baldur's Gate would be wonderful to have in that kind of format.