Allen should go right around the Top 10 regardless of what any draft sites think. He's a flawless receiver that played with a terrible QB. With the Jets, Vikings, and Dolphins picking in the 10-15 range, he won't escape to the late first.
I do really like your idea of finding an outside receiver and moving Decker to the slot as Stokley's heir apparent. Decker can't catch deep passes anyway, but he's pretty good facing the QB, so that makes a ton of sense to me. If we're talking late first round possibilities, Terrance Williams or DeAndre Hopkins would make sense.
I think we go MLB in Round 1 though. Elway believes in immediate impact starters early in the draft, and I'm not sure Stokley retires, and even if he does, if Tamme simply doesn't become the primary slot guy. So, not sure a WR would be impact enough for Round 1 this draft.