Game mechanics question.. Please Help

So far i have made my map but haven’t started implementing AI or anything. I need some advice first. Also my game is a open world shooter so its a large map and there is a lot of things in it such as AI, looting, vehicles, etc… however its only going to be multiplayer so there arent any game modes.

So I wrote down all of my ideas and the whole layout of the game before I done anything. Looking back at it now it seems like a lot and Im wondering how hard its going to be to add everything. For example, If you choose a certain class then you automatically have access to certain skills. Also were my game is only multiplayer how hard is it to add the feautre were individual players inventories save? Would something like this be hard to implement into a game. Im a beginner but Ive been learning fast, im curious to know if I should add every detail i wrote down or just simplify it because it will be too hard to add it all. Any help will be greatly appreciated, thanks!