Idea

Currently there is a "Fade Duration" setting for each frame (and global) which is used for both fade in and fade out. If these could be made separate for fade in and fade out that would be great. I personally prefer a really quick fade in (0.25 sec, for example), but a really long fade out (5 sec, for example). This feels better and reduces the pop-out feeling when exiting combat.

In the current state this would be a rather big change. I'd like to wait and see if this will be requested more times.

+1 for this. Frames need to appear near instantly for times when you have to react quick. Fading out shouldnt be instant for clean & immersive gameplay. It is very important to me and I hope you reconsider adding this.
Putting 1 toggle for both means I have to sacrifice 1 to have another. It sucks and yet there is no other addon to fill the void. They all cut corners on things.