I know it's possible, but I'm not sure if there's a reason it's not already implemented or desired.
Toggle-able feat based abilities, such as Expertise, Power Attack, Knock Down, etc. It's incredibly frustrating to try to trigger these abilities when you mean to use them, or just unreliable in general. For example, I might click on knock down, yet it may take 3-4 rounds before the attack is even made. Having it always up (at -4 or -5 ab or what have you) is preferable to me
Edit: Revanant got it it one. There are a LOT of things that just drop you out of your combat mode or trigger at wierd times. It is already possible to queue things such as knock down, or move while under expertise. Its just finnicky, and often irritating.
Implementing a small change would be a huge Quality of life improvement, in my eyes.