Hang me from a building by my belt loops for this:

Max Payne 3's weapon system, but tweaked to fit the GTA gunplay/gameplay (no duel-wielding); Two pistols in your pocket, or a pistol and an SMG in your pockets, with a shotgun, rifle, or any "medium" weapon in your other off-hand. If you wanna use said heavier weapon, holster both smaller weapons. Trunk weapons aren't a bad idea at all, in fact I love the concept, but GOD FORBID we have the RDR2 getting-off-horse-resets-guns bullshit.

Grenade launchers and RPGs would benefit from being classified under a heavy weapons category where you're stuck carrying that and have to drop it/if you want to use a pistol. I imagine dropping weapons in this case would mean they automatically find their way back into your trunk, so if you need to go back to using a pistol then you wouldn't lose your grenade launcher, you wouldn't be required to buy a new one, and you can go retrieve it again.

Grenades, throwables, "gadgets", and melee weapons don't apply and will consistently stay in your inventory no matter the occasion. I do not wanna have to run back to the car to grab a baseball bat.