With Dragon Age 4 in development, fans have had a lot of time to think over Solas' plan. In Trespasser, he could reveal a lot to the player, such as his plans to tear down the Veil and his spies within the Inquisition. It is also likely that he only divulged a tiny part of his plan, as he does frame himself to possibly be the main villain coming into Dragon Age 4.
Tearing down the Veil is a lot to comprehend. It will change Thedas in many ways, and there are secrets locked behind the Veil just waiting to be discovered. Once all is considered, a torn-down Veil is just the tipping point of much bigger issues for Thedas. While the Veil getting torn down by Solas is bad, what will happen after will likely be much worse and more complex.
What Tearing Down The Veil Means for Dragon Age
In Trespasser, Solas mentions that tearing down the Veil will require him to sacrifice the current world to save the old one. In discussions by fans, a lot of theories believe magic will come to all people in Thedas, or perhaps just the elves - who will also regain their immortality. According to Solas, the Evanuris are sealed beyond the Veil as well, so they will be free.
As for the demons and spirits that live in the Fade, it is unknown what will happen. Demons have thirsted to cross over, and with the Veil torn they finally could appear in the physical world without possessing people. There could also be a lot of spirits that end up like Cole, and the Old Gods are believed to be in the Fade as well. Some theories believe the Old Gods and the Evanuris are one-in-the-same, but there is the possibility that these Old Gods truly are powerful dragons.
Whatever the case, Solas lays out that tearing down the Veil will destroy the world, but he does not go into detail. It may look a lot like rifts in Dragon Age: Inquisition, except on a far more catastrophic level. In terms of what can happen after the Veil is down, there is a lot to digest.
The Possibility of Two Blights At Once
There are two known Archdemons that have not been dealt with by the Grey Wardens. This means Thedas can expect two more Blights. With Dragon Age 4 approaching, fans have begun to wonder if two Blights could happen at once. This possibility seems likely in the event of a fully torn Veil, especially given Solas' words about the world being destroyed due to his plan. This is also evidence of why Solas does not like the Grey Wardens, because they are a threat to his long-term plans of destroying Thedas.
The possibility of two Blights is supported by the fact that the Black City will be accessible without the Veil; where the first Blight and Darkspawn began. Another theory says that tearing down the Veil will cause earthquakes due to the amount of powerful changes. If that does happen, these earthquakes could free the two last Archdemons from their prisons. There is also the possibility that releasing the Archdemons is not part of Solas' plan, but a possibility he can't control that he's still willing to set loose.
The Evanuris
The Evanuris have become a subject of great interest to fans after Trespasser. The elven pantheon turned out to be a group of powerful mages, and it was revealed that Mythal is a part of Flemeth while Solas is the Dread Wolf. With these serve as examples of how powerful Evanuris can be, there are a lot of reasons to be concerned that Solas is setting them loose.
If the Evanuris are anything like Solas and Mythal, it is likely they have their own plans. Solas has a track record of failed plans, and failing to take care of the Evanuris would be devastating to Thedas. The reason he sealed them away in the first place was because they were power-hungry, cared not for their followers, and were murderous. It is likely that the first thing they will do if released from the Veil will be killing Solas. Based on what has been revealed about the Evanruis, Solas would be a preferable enemy.
The Black City
The Black City is one of Dragon Age's greatest mysteries, and with no Veil the city should be accessible again. Most of what is known about the Black City is through the Chantry, and fans have a lot of reason to believe the Chantry has gotten a few things wrong. Corypheus has mentioned the Black City and its throne were empty. Some records say the Old Gods whispered to humanity from within the Golden City, while the elves believe their gods are trapped there by the Dread Wolf.
The most worrying thing about the Black City being accessible is that it is the center of taint. It is where men were made Darkspawn and the first Blight was triggered. This taint could possibly spread throughout Thedas without the Veil holding it back. In fact, some fans believe the taint was created by Evanuris trapped in the Black City. Whatever the case is, a lot will be revealed if Solas tears down the Veil and the Black City becomes explorable.
Dragon Age 4 is in development.