Demaryius Thomas is No.2 in the league for 20+ yrds catch (Calvin Johnson is #1). This is the guy who was made by Tebow (according to some of his fans)
His development under Peyton this year is staggering. And he just started to scratch the surface. He and Decker would be even bigger threats next year. They haven't peaked yet.
DT is a monster and probably the most potent WR Manning has ever worked with. If Manning can make Marvin Harrison one of the best ever, I don't think he will have problem making DT even better.
I don't believe these tall and physical WRs have been used correctly in the league, even including guys like Calvin Johnson and Brandon Marchall. Their QBs only throw the ball to their direction and let them make the catch regardless coverage. Had they been working with Manning they'd be even more productive.
If Broncos could add another DT on the opposite side through FA or draft and move Decker to the slot, this could easily be the best WR group in the entire NFL.