Racing track, every map needs it and doesn't have it, call it Blitzspeed/Blitzspeedway As if lightning speed for them boys. Unless blitz is war, then idk what you'd call it lol. Anyways you got to include this mate, this'll be very original and it's almost never included in RP city maps. Good luck on the build! If I think of anything else I will rush it over here!
You should go for something unique maybe even a mall but here are my ideas.
Forest with ranger station
Fishing lake
Subway system?
Rich area complete with pine trees and some expensive houses.
City square maybe with its own clock or fountain.
A dench Fire House with a Chief's office. If might have a common room, officers quaters (for the Chief) and the garage with like 2-3 gates and a tower to dry the washed hoses.
Have an area on the map where its filled with luxury mansions. People could then show off how rich they are in game
(It'd also be perfect for growing some dank kush)
The Police Department has been added with an underground car park from the request of @LordTyla - A park has also been added, soon with a Bakassi Statue thanks to @Belg Lmfao.