I take a different view, I don't think he was a member of the pantheon originally, but either a creature (i.e. created by Mythal or Ghilly), one of the forgotten ones adopted by Mythal, or something else entirely…
I am mostly inclined to believe that he was one of the forgotten ones and helped imprison them in the abyss, thus gaining the favour of Mythal and when things in Arlathan began to go south, he decided that the time had come to take action again, perhaps even with Mythal's help and approval.
Well in Dalish tales it does say that Fen'Harel was neither god nor forgotten one. But the thing is, the Dalish have an extremely skewed view on their history...but it still does kind of make sense that in order to accomplish what he did he would have to be neither those things. To have the view of freeing people and overthrowing the pantheon(s) he would have to identify with the common people. Which I don't think any god (or even forgotten one) would do. To lower yourself to what the elven gods considered scraps and expendable materials...he would have had to either dissown his godhood, or not be part of it to begin with.
Which also makes sense that in order to overthrow them, he would need powerful allies...as made clear with his banter with Sera "why not take it all the way?" which means that he intended to replace the gods with a newer "better" government perhaps?
So then is his plan to release the gods? Now it doesn't seem so likely...
GAH my thoughts are everywhere!!!