Transparency Is A Huge Issue Currently for building a 2D, 2.5D mix with 3D game.
I have tried to document what is going on over at answerhub here:
I really want to build something to the quality level of Rayman Legend or Ori and the Blind Forest, but currently the way transparency is handled in UE4 I think that just might be outright impossible.
I know that there are strides being made with paper2D in UE4, but this is currently very limited, especially with the set up that I am trying where you essentially make a 2.5D puppet and animate it to get your final look.
It would be great to hear a firm commitment on Epics part to ensure that we could build these types of games in UE4 along with the beautiful 3D art that is currently possible.