Also zero evidence that Cory is the one behind it.
According to the Masked Empire the prep for this has been going on for far longer than 3 years (the minimum amount of time Cory could have been freed), and why would elves be working with a Magister?
There isn't much evidence as to who the Elder One is.
As to whether or not Cory is the big bad of DAI, here are the indications that come to mind (I welcome anyone to add to it):
- He has influence over the Grey Wardens as shown in Legacy. We know the Inquisition attacks the Grey Wardens from the trailers. The devs has said the Inquistion is looking to find the source behind all the chaos that is happening in the world and Orlais which seemed too convenient to be all occuring at the same time. Putting two and two together, Cory is either working with the Elder One or is the Elder One himself.
- The Elder One is described as relentless, ambitious, beyond arrogant, with a massive God complex, believing he operates at a level no other being can comprehend. Sounds like someone we already know: Corypheus.
- There's too much 'build up' of the ancient magisters who stepped into the Golden City and started the first Blight and beyond. These are clearly indicate the magisters hold significant importance in the DA world. Would Bioware passed up an opportunity to use one of them as the main villain of their games? Actually, I think it was planned all along.
- The World Of Thedas disproportionately emphasizes Corypheus' role more than Hawke or the Warden.
- Let's look at the candidates out there as to who could be the big bad boss in DAI: Flemeth, Old God baby, Fen Harel, one of the Forbidden Ones, Corypheus or a yet unknown powerful being. Flemeth is up to something that is brewing for a long time. There's no other entity in the world of Dragon Age that was given more build up than Flemeth herself, so she could be a prime candidate. Old God baby? He appears as a young boy as per one of the leaked art recently, I find it hard to believe it is him. Fen Harel appears a lot in the codex entries of DA, and could well be the person who killed Felassan in the Masked Empire. It could be him. But let's move on to the next. One of the Forbidden Ones - these are powerful demons, but we know Imshael was released in the Masked Empire and found out from Felassan that there will be a great deal of violent upheaval to come, so it's unlikely to be him. Xebenkeck and Gaxkang have already been killed in DA2 and DAO respectively. That leaves only the Formless One among the Forbidden Ones. So between Flemeth, Fen Harel, Formless One, a yet unknown enemy, and Corypheus, who do we choose? I'd straight up put Flemeth away (together with Morrigan and her Old God baby) as I believe she will play significant roles throughout the Age of Dragons and future games. So among the four left - Corypheus stands out as the one who had the most recent attention and 'build up', therefore the prime candidate, compared to other three who had almost none. Legacy clearly foreshadows him as a returning character in future games.
All these are not evidence, but they are educated and reasoned guesses. In the absence of real evidence, I'd go for what we have on the table, and what we have on the table so far points to Corypheus as the likely big bad in DAI.
But the truth is, I do have evidence in my hands. So I'll open a potential can of worms that I know many will not believe, but it is true. In 2012, I received significant information about DAI (back then it was hypothetically called DA3) which was accidentally leaked from Bioware - I'm likely I'm the only person outside Bioware who has this information. There was a major leak back in (I think) back in 2012/2013 everyone talked about; this leak was not that. It contained information that overlaps with that well-known leak, and others that that the well-known leak doesn't have. These are early development details - guess who was mentioned repeatedly as the big bad of DAI? Of course, the villain may be changed throughout development from then until now, but I'm pretty sure it remained the same as it was back in 2012. We have a month to go to find out for sure.
I'm 99% certain that Corypheus is the big bad in DAI. You mentioned in Masked Empire that the event in DAI has been planned for a long time - that's certainly interesting and I'll look it up, hope for further discussion. It's worth noting that Flemeth has foreseen so much that has happened in DAO, DA2 and even DAI, so there may be other powerful entities (Flemeth being one) in DA world who knew all these are forthcoming, including the master of Felassan.