Depends completely on who does what during the offseason.
Maybe the Bills stumble upon a competent QB, with Watkins/Woods and Jackson/Spiller they have some solid skill players, just need the all-important QB. The defense will be very good.
Maybe the Broncos decide to retool for another run, and Manning's able to rebound from his late-season issues. They've been good before, and they've beaten the Pats before.
Maybe the Colts solidify their OL and front seven, time will tell whether they can play with the Pats this week but they don't need a lot of changes to become a mighty scary team. A little more punch-you-in-the-face types on the lines and they'd be a complete team.
Maybe Pittsburg spends all their resources on the secondary; their front seven is generally solid, their offense has home run hitters across the board, they just need an upgraded secondary to become serious threats.
Maybe the Ravens retain their current talent while getting healthy and/or solidifying their secondary in the draft. It's already a scary matchup, if Melvin wasn't exposed on Saturday we'd be talking about what the Pat's need to do to contend, not the other way around.
Maybe Miami can find some more help in the secondary and Tannehill takes another step forward. They beat the Pats once this season, their front 7 are legit and they have a solid mix of skill players on offense. They're not far off, imo.
The Texans need to fall into a QB, same as Buffalo. Their defense is fine, solid skill players on offense, they're a QB away from being a major threat.
Any or all of these things could happen, with varying degrees of probability, and I'd consider that team to be a threat to the Pats over the next 2-3 years. Most likely are Ravens/Colts/Steelers, imo, with Miami and Denver not far behind. The teams that need a QB...good luck! I don't see any of them becoming Seattle-esque juggernauts, thankfully, but I fully expect 3-4 teams to emerge as legit Super Bowl contenders from the AFC.