I think it would be really nice if there was a "City Builder" genre or possibly a one-step broader "Overseer" genre. (Not the best name, but I'm sure someone can find a better one.)
I understand that city builders currently fall under the "simulation" genre as a whole, however my arguments are as follows:
1) City Builders/Overseer Games are a specific enough subset of the simulation genre that someone who is looking for games similar to Sim City, RollerCoaster Tycoon, or Tropico is probably not going to be interested in also looking at games like My Summer Car, Football Manager or DayZ. Even using themes to narrow the search to "Management + Simulation" still returns games as diverse as Euro Truck Simulator, Battle Chef Brigade and Recettear in addition to the more expected games like Cities:Skylines,
2) The amount of City Builder/Overseer games are large enough to be worthy of their own genre category. Considering that Tennis (149 games), Skateboarding (89 games), and Track & Field (46 games) all have their own specific genres, I think City Builder games are easily able to get into that number, and probably even higher.
3) There is already precedent. As noted above Tennis, Skateboarding and Track & Field all have their own genre categories despite ostensibly falling under the existing "Sports" genre. Likewise "Card Game" (arguably a subcategory of "Board & Trivia games), "Flight Simulator" (an obvious subcategory of Simulator) and MMORPG all have their own specific genre.
4) I think the genre is sufficiently well-defined, so people should be able to easily decide whether most games would (or wouldn't) belong in the genre. If pared specifically to City Builders, it has already been decided by common usage: games like the Sim City, Tropico, Anno & Cities franchises. If expanded to "Overseer" games, it still is usually understood as "top down/isometric games with a disembodied overseer who is able to place things at their whim and who is responsible for the needs of the people on their limited map." This would expand the genre to games like Dwarf Fortress, Transport Tycoon, Prison Architect, Dungeon Keeper, SimTower & Roller Coaster Tycoon. At that point the edges of the genre do get a little muddier (would games like Supreme, Crusader Kings & The Sims count? My feeling is No, No & Yes, respectively.) but I think that for the most part it still passes the "I know it when I see it" test.
Thank you for taking the time to consider this.