• Not a crater town (aka mountain walls all around the city.) For
example, back when I built the map for Crown, 1/3 of the outer edge of the city was mountain. The rest was open grass hills or ocean. Use invisible baseplate cubes (or even terrain blocks) to keep people blocked in. For extra security, add a layer of zone bricks behind that. Kill any player and destroy any vehicle that enters that zone brick.
• Administration =/= job. When admins get ridiculous amounts of money on City RPGs, it ruins the balance. You get a bunch of admins that are either ONLY building, ONLY killing, or they quit after less than a week, simply because at that point they've already "won," or as close as you can get to winning on a City RPG.
• Constant development should be a thing. Cities don't just start out with a stuff ton of empty lots and a couple of official buildings, they start as a town and move their way up. As more space is used, place down more roads and expand outwards. Eventually, start a new town.
• No redundant weapons. It's a City RPG not a COD City TDM. No explosives, no automatics, no insta-gib/kill, etc. Statistically, criminals use pistols in crime. Police carry pistols/stun-guns. If you think you can keep it balanced, SWAT and gang members are really the only people who SHOULD have rifles/shotguns.
Those are kinda just the biggest things that bug me on City RPGs