I know you can set an alpha on a material, but I would submit to you that is an overly complex way to fade out/in an actor. I would also submit to you that fading actors in/out is such a basic operation that every Actor should have a transparency value built into it.
I’m not sure if this is a big deal, but it certainly would be a big time saver.
Thanks for your consideration,
This isn’t possible for a number of reasons, but the first and foremost is that every shader would have to be translucent. That would very literally destroy performance on every platform.
You can’t render an opaque material as a transparent one, it just doesn’t work that way.
One thing you can do is make all your character materials masked, and then use a screendoor dither fading pattern to kill certain pixels. This is what Uncharted 4 does to fade out things between the player and the camera. Making all your character materials masked has a performance impact, but not nearly as much as making them translucent, and all the deferred shading features continue to work.
In summary, this is the type of thing you should implement yourself using the material editor and blueprint to set parameters on the material.