Going down a route of being able to kill just about anyone, including importent quest givers would need the devs to make some kind of system where you might still be able to get to the main quests things. Backup ways for the game to give you those importent main game missions.
Like if you kill a quest giver and loot them you might find a datapad or something which gives you some kind of information that opens up the quest. And/or if you did not loot the quest giver you will some time down the road maybe get a message on your phone/email-type of equivalent thing in the CP2077 world, where someone maybe says somehting like "It has come to our attention that X person is dead, and that you killed them... we would like you to... etc etc etc".
Of course... it should not be just that easy... kill important npc and get away with it... no if you kill someone like that then you should get some kind of penaltis for it... be it in a lot more enemies in the mission area, or a major reduction in payment for the mission, lose reputation with X factions (although, you might gain reputation with others), possibly have you hunted by the law enforements and/or faction the quest giver came from, and maybe even suddenly finding your self hunted by an unknown person who takes potshots at you and disappear befor you can do anything about it... which might cause problems with where you are since it might draw attention to you by law enforcements or the people who live there, etc.. and might be really bad if you are in a sneaky kind of a mission at the moment... it might even if your unlucky (due to maybe already being seriously hurt) kill you... that person might in the end turn out to be a familymember or loved one of the NPC which you killed, etc.
Like if you kill a quest giver and loot them you might find a datapad or something which gives you some kind of information that opens up the quest. And/or if you did not loot the quest giver you will some time down the road maybe get a message on your phone/email-type of equivalent thing in the CP2077 world, where someone maybe says somehting like "It has come to our attention that X person is dead, and that you killed them... we would like you to... etc etc etc".
Of course... it should not be just that easy... kill important npc and get away with it... no if you kill someone like that then you should get some kind of penaltis for it... be it in a lot more enemies in the mission area, or a major reduction in payment for the mission, lose reputation with X factions (although, you might gain reputation with others), possibly have you hunted by the law enforements and/or faction the quest giver came from, and maybe even suddenly finding your self hunted by an unknown person who takes potshots at you and disappear befor you can do anything about it... which might cause problems with where you are since it might draw attention to you by law enforcements or the people who live there, etc.. and might be really bad if you are in a sneaky kind of a mission at the moment... it might even if your unlucky (due to maybe already being seriously hurt) kill you... that person might in the end turn out to be a familymember or loved one of the NPC which you killed, etc.