It seems likely to me that Suh will hit UFA. The Lions basically can't franchise him, and that means that can't threaten to franchise in negotiations. Also, and this is pretty significant: Detroit.
The fact that the Lions are attempting a trade is a pretty big clue that he isn't likely staying with the Lions much longer.
Even with the hit to Suh's image and his inconsistent production, he's just such a rare player that I expect interest in him to be pretty strong in free agency. It would be cool to see him end up in Seattle but for anything more than Michael Bennett money I don't think we could budget it, unless Russell Wilson signs for less than expected.
In a trade though? I would be surprised. Maybe a team with money to burn like Oakland might make some sense as a trade partner, but for most teams he's just going to be too expensive.
Suh may not be perfect and he may be overpaid but Detroit needs more players like him, not less. The Lions are already the NFL's most finesse team even with the league's dirtiest player anchoring their defense.