At the very least, we'll probably get to hunt and kill it. And the quest might give some answers as to what their general deal is. The Forbidden Ones were originally banished by the Evanuris, so maybe that'll figure into it.
There's one Evanuris lurking around in thedas with the plan of bringing back the rest of the crew; maybe the formless one will be the rock in Solas' sole. Getting revenge on those guys once and for all.
What if we are the Formless One in the next game? We sensed the impending cataclysm that Solas would unleash, took a mortal form and brought ourselves into Thedas. We wiped our memories and have to piece them back together to further the story.
After Darth Revan, this plotline was burned. I really don't want to see it again, because in KOTOR worked just perfectly.
I think if Imshael had more sequences to his fight, he would have been a lot more interesting. For one thing, he uses hexes in the book. So why couldn't he have used abilities that make the characters weak and vulnerable to certain elements? Also, it would have been cool if he stayed as a mage for part of the battle, then drew everyone into the Fade for the rest of it where he would fight in his demon form. Just that simple change of scenery would have made the fight interesting. Another ability he could have used is marking the floors with glyphs. Perhaps even ones that move around. This would require constant dispelling from the mages in the party or Cassandra or a templar Inquisitor.
Well, blame the game devs for it, since they were the ones to remove the hexes and the entire Entropy school from the game. It was really cool to use the death hex alongside the misdirection and vulnerability ones on Loghain. Or when using mana clash, which was the easiest way to dispatch Gaxkang. Although it was also annoying when it happened to you... The fight with Imshael suffered from the same as all the fights in Inquisition; it was limited by the button-bashing nature of them and the less ammount of variations that foes had. It would have been much better if he fought as an Arcane Horror, since in nightmare they are the only really though foes with that terrible beam of spirit damage they unleashed... The glypsh wouldn't have helped at all I think since potentially all four party members can dispel or fight from afar. Maybe add some archers with good skills like the annoying Avvar archers that can get you easier than a dragon.
I think the Formless One is mentioned by Solas, he says once he met a powerful demon, but when he asked what he was, the demon said it has been forgotten a demon with no defined purpose would perfectly fit with that name.
Or maybe it was the embodiment of some emotion that only the ancients had... Although I have the notion that Solas is the formless one, after all he is a rift mage, the spells of that tree are described to have some relation with the void, which was where the Forbidden Ones were banished, but maybe it was the forbidden ones who banished the Evanuris.