I think that indeed all original bosses will become awakenable.
So maybe the next will still be Wrath followed by the Killer Beast.

Raven Witch as being the first boss you meet is even newer then the Killer's and was actually the first non-LOL that was awakenable as far as I remember.
Wrath is the only end boss however that is also a part of the Eternal set and that's why there is a Demon Prince Variant.

There are still only 2 awakenable Ed's however.... Thor and Cyborg...
So we can extend the question here: Who will be the next awakenable Ed?

I think it will be POM or Seventh Son, where Vampire Hunter might be planned for next Halloween.