Here’s a few I’ve used that have worked well:

• Three Sided Battle: The party encounters two different groups of NPCs fighting each other. They can join up with one side, but there will be consequences if either side is left undefeated.
• Weak Spot: The enemy can only be damaged by attacking it from a certain direction or by targeting a certain part of it.
• Two Health Bars: An enemy has two forms that it can swap between at will. Each form has a distinct health bar and if one of the forms is defeated, the enemy is locked into the other form which then becomes more powerful.
• Mirror Match. Is there a really powerful ability a PC has that everyone has been enjoying on their side for the whole campaign? Give it to an enemy.
• Tactical Enemies: Certain formations give the enemies massive bonuses so the PCs need to break apart the enemy formations.
• Hiding Spots: There is an area where the enemies are safe from harm and the players must figure out a way to draw the enemies out.
• Death Zone: There is an area highly dangerous to the players, such as lava or acid, that doesn’t harm the enemies, and the enemies’ strategy is to drag the players into the death zone. Can combine with Hiding Spots as well.

I hate how rarely you get ASI’s and feats. Plus with the reliance on half feats, it’s so much work to figure out what to do. I think that all feats should change to half feats, lose the built in ASI, and then every even level you get a choice of a +1 in a stat of your choice or one of these ASI-less half feats.