Reese and Cyrus aren’t on that stage because they are from New Leaf and the Town and City stage is based on City Folk.
It is possible that they could appear as an Assist Trophy, but not appearing in the background does nothing to hint that. As we’ve learned with Toon Link, one can be in the background AND on the battlefield.
But about that Duck Hunt stage.
The Duck Hunt game for the NES is very iconic, known for the NES Zapper gun included with the game (which would make a good item perhaps?), and yet Smash Bros. has yet to acknowledge it further than a trophy in Melee. A stage would feel right at home for this series.
Hatters suggested an HD design, but I have a feeling Sakurai would want to preserve the original sprites, as he does with most retro appearances. In any case, Duck Hunt is a series I’d like to see in some way. If not just to hit that smug little dog’s face.