That’s so great what you are still active and is interested in improving and moving your project forward! :))
I’d say pixel creep [done] + per-object outlines + per-object highlights are the minimum for a solid asset, well, with transparency working well too. After that… I actually think what both plugins I mentioned before are pretty feature complete. There is only some stylization options to be added on top, like curved CRT matrix, different dither, and so on, but that’s a low priority, thou can add additional visual frare and to the priceiness 
Good luck! And thank you.
P.S. As I mentioned, I came from Unity background, because my current (first) game kinda requires a little better graphics and is first person POV, but doesn’t require much coding, so I made a risky move to try to jump to UE temporarily. But I’m quickly falling in love with UE, and probably won’t be back to Unity 
And this was the asset I’d miss in this case