I'm aware that editing your inventory can be done in game, but you lose all the changes that have been made and have to recustomize everything each time you respawn.
I've seen some mods that were made to make custom loadouts(yes i've seen Dslyecxi's video and a few others that were similar), but as far as I know they only work for editing npcs for the map editor or single player missions. I think there should be some way to customize and save/load your own inventory loadout either before entering a multiplayer match or alternatively letting players save their own inventory in game and then giving them the option to load it when they spawn in or respawn.
Some hosts may want players to use certain loadouts for specific scenerios and I can understand that, however I am just suggesting that the option should be available(atleast with hosts that allow it) for players to be able to use their own custom loadouts when entering multiplayer.