Here are some ideas for Decals that I personally would find useful.
Decal Channels- Actors can opt in or out of certain Decal Channels. If Decal Channels are not possible, maybe we could get the option to have actors accept moveable or static decals again. I’ve noticed weapon bullet decals no longer show if Decals are disabled on an actor in UE4.
Decal Modifiers - Some modifiers like bend, warp, taper. For example if you had solid line decal, you could add a 90 degree bend to it, which would let you use the same decal to curve around a corner on a floor.