Besides Solas as an antagonist if he fills that role in DA4 is overpowered.
We've never faced anyone like that honestly.
Here's a guy who can turn you to stone with his intense frowny stare , is probably the best dreamer alive so can take a look in people's mind if he's so inclined or ask spirits for informations , has a network of Eluvians all over Thedas , and let's not get into history and magical knowledge lost to time ...also is kind of paranoid as he doesn't truly trust anyone but himself.
So the "The Inquisitor can't do it because Solas knows him/her." Errr...problem is no one can actually face/trick /spy on Solas right now , you could maybe get a few elven spies in his group but I doubt he'd tell his plans to a random current elves , and with him leading a war against Evanuris in the past ,etc he probably is intelligent enough to separate elves in different cells and gives them only the information they need to perform whatever.
So Inquisitor or new random hero will need an edge , help from random ancient being , or random super magical artifact /relics to tip the balance anyway.