Knockouts. It’s a very simple rule. If you successfully sneak up on a creature and successfully hit them with an attack that deals bludgeoning damage, they need to pass a con save with a DC equal to 10 or half the damage dealt (whichever is higher) or they immediately fall unconscious. I feel like 5e doesn’t really encourage stealth enough otherwise, and this way players have a decent way to pick off sentries and such before raiding a camp.
In theory you could do assassinations with a similar rule, but I haven’t tried it out in a campaign.
I think feats are one of the coolest aspects of D&D character customization, so it’s always a pain when it’s stifled in favor of number-go-up. Depending on how high I plan to go, I give my players a feat every level or every other level. They end up with more than they know what to do with and end up taking RP feats, which are sorely underutilized because of the opportunity cost of obtaining them.
Yes, the PCs get more powerful, but that just means I rebalance enemies.