I agree that Knox might take it. I guess I've just seen WRs have such short careers here, and productive ones aren't around for very long, so that makes me skeptical to think that Diggs will be here for all 4 years of his contract, or a rookie (this time Davis, but countless others in the past) popping up in camp and looking promising only to not contribute all that much.
Knox feels like he can chug along at 45-55 catches through the next 6 or 7 years or so, so he might be the guy.
BTW, the 5th year option is only available to 1st round picks. Knox was a 3rd rounder, so there is no option for him. If his next 2 years pay off any of the promise he has shown so far last year, they surely try to give him a new contract before his 4th year starts. We can't afford a franchise tag on a TE, as that market has blown up this offseason.