The only mechanic I like is size of equipment affects your stealth abilities (and preferably skills and speed movement). How much attention you attract or noise you make. I'd love it implemented in the game.
Getting back to your car to get a bigger gun interrupts gameplay. Especially if it happens on regular basis. It's hard to inject this mechanic into quests, because I bet many quests will have isolated environments where you can't reach you car till either you're done or the quest is cancelled. In L.A. Noire it worked because it was super limited and because it's tied closely to mission progress. Another issue to consider: if you swap a car, what happens to your trunk stuff? Is it miraculously transferred to the new car? Anyway, I believe usable trunk brings more issues than improves gameplay.
Getting back to your car to get a bigger gun interrupts gameplay. Especially if it happens on regular basis. It's hard to inject this mechanic into quests, because I bet many quests will have isolated environments where you can't reach you car till either you're done or the quest is cancelled. In L.A. Noire it worked because it was super limited and because it's tied closely to mission progress. Another issue to consider: if you swap a car, what happens to your trunk stuff? Is it miraculously transferred to the new car? Anyway, I believe usable trunk brings more issues than improves gameplay.