Worst case scenario it might end up like BF2 did back in the day where the DLC stuff was way overpowered. A sort of "sure you dont have to buy the Kill-o-matron 4k Laser Minigun DLC....... but you will be screwed over by everyone who did" kinda deal.
The other possible problem is that, as mentioned by others, the DLC guns are simply not "worth it" and people wont buy any DLC. While I was not very fond of BF3 I kinda like their DLC system where they released larger DLC packs instead of just small mappacks like with CoD. Maybe that would be a solution. Instead of mappacks just release DLC with say a couple of new maps and weapons and maybe even new gamemodes if possible. A sort of "upgrade" pack rather then just new maps.
One thing I could see though would be alternate model DLCs for weapons. Like for an example a M4 model for the Honey Badger or a AK-47 for the AK12. Basicly the same gun statwise but with new looks.
This isn't a new request. I remember people asking for weapon DLC all the way back to MW2. I have mixed feeling about weapon DLC. It's really such a difficult thing to get correct. Say they charge for weapon packs or individual weapons. If the guns are too powerful then people will complain and call the game pay2win. If the guns are crap the people that bought them will complain. The only way for them to make everyone happy is to have free weapon DLC, but let's be honest it's Activision, that won't happen.