First of all, skills should not greatly unbalance the game
They should reward players to play more

If a player does not play for a long time, for example, more than a month or three, then his skills deteriorate

Example of skills:

1. Weapon Reload Boost (You often play as a sniper, now your sniper weapons reload faster)
2. Accelerating the installation of weapons on bipods
3. You often crawl on foot, now you can do it a little faster
4. You often play, so your character is used to the current weight. Now you can wear more
5. etc