Ok Just as the title says:
Add a bunch of NPC from storyline and make them spawn in their usual locations from story and they can give you missions when you go speak to them.
Idk you can add Cesar Vialpando/Madd Dogg/Sweet/CJ for Los Santos.
Zero,T-Bone Mendez,Jizzy B,Wu zi Mu for san Fierro
Kent Paul, Ken Rosenberg,Mike Toreno for LAS Venturas.