This is highly unlikely. If, and only if the situation were right would they do this. There is no point in doing this if you still have Ben Roethlisberger on your roster. If he were to retire after this season, then yes, it would be the perfect situation for Watson and the Steelers where he would find a new home and a stable franchise where he would have the weapons to compete instantly.
The price would be high for the three-time Pro-Bowler; however, the price would be worth it after Big Ben. He would be the instant answer rather than having to wait to find one in the draft. There would be no limbo period. The Steelers would have a franchise quarterback.
The price Houston might require for a player of his caliber would be multiple First-Round picks and perhaps a player within the trade as well. If I am the Texans, I would not even consider trading him. I would try and mend this relationship because he is too good of a player to just trade away. He is the type of player you want leading your franchise for years to come. However, it looks like there might be a messy breakup looming ahead for the pair.
The only way the Steelers find a way for this trade to work is if Ben is gone and his cap hit is gone. Watson would also only want to go to a team that makes sure he is the starter from Day 1. He is earned nothing less than being the guy.
In his career, he has been to three Pro-Bowls and thrown for 104 TDs and 36 Interceptions. This is all achieved in just 4 years in the NFL.