"Who would stand to gain the most from the Fade becoming one with Thedas?" is the question we should be asking ourselves.
In my opinion, a dreamer would gain the most. Probably an ancient one. A dreamer can manipulate the Fade to his or her will, so the Fade and Thedas becoming one would enable a dreamer to manipulate all of reality.
This might explain why
was murdered in
The Masked Empire, in order to remove
the "competition".
That particular character seems to have been a very powerful and aged individual, if I'm not mistaken.
I don't think the Elder One is Solas, though.
Ok i just came up with a theory. ok listen to me here this is far fetched but hold on.
So Felassan is a dreamer. and we can all agree he was either killed(or something close) in or out of the fade. and we learned in DA2 that demons wanted to posses a Dreamer because they could manipulate the fade and become incredibly powerful. what if Fen'harel, or who ever it was at the end, took control of Felassan so he could manipulate the demons in the fade to obey his command and attack the human world instead of just rushing out of the breach and going where ever they wanted.
I mean people keep asking what would Fen'Harel want though? He locked the Gods away in the legend so he come roam around freely, it seems in his character to take control of a dreamer, and through concept art we know we might have to go to apart of Arlathans, and then we see this elf in the trailer (somewhere that kinda looks like where the concept are was drawn) bringing something out of the water, and we know that big bad at the end of TME wanted the eluvian network.
I know this is completely far fetched but it just bounced into my head.