In my opinion, the best way to go about a unlocking system is with a tiered token system. Every level up, you get a token and can use that to unlock a weapon or gadget of a tier.
For example, tier 1 would be AK 15, MK20, USP, whilst tier 2 is SVD, Honey Badger, RPG Tandem etc.
Higher tiers get unlocked with higher player levels. For example, Tier 2 would be unlocked from level 20 onwards.
So people can go for what they want, without it being fully free and people able to go to the meta options immediatly. If you want to focus on ARs first, you can do that etc