A lot of this will change based free agency and the draft, but here are the teams that could unseat Seattle with the roster they have now.
Green Bay: Had numerous boneheaded mistakes in the NFC championship. Had anyone of the mistakes made in the 4th not happened the Packers probably could have been the ones in the Super Bowl last night. They do have some key contracts expiring so this may depend on if they can keep guys like Randall Cobb and Brian Bulaga around for next season or not.
Dallas: They were 1 bad call away from going to Seattle, which was a place they already had won in earlier in the season. Romo and Murray were having some of the best seasons of their careers as well. A lot could change if Murray and Bryant walk in free agency though, so the jury is still out on them.
Arizona: Did anyone have worse luck than Arizona did? Carson Palmer isn't elite by any stretch of the imagination but was playing well until he went down. They probably could have survived with Drew Stanton, but once he went down the season was officially lost. If they didn't get bitten hard by the injury bug then it's very possible they would have won the NFC West over Seattle.
Philadelphia: Another team hurt by injuries. Starting Mark Sanchez was worse for them than losing Nick Foles imo. Foles isn't the greatest QB either, but the Eagles looked like contenders before Foles was injured. If they can keep Maclin they may have something next year
Something to remember is that before the Eagles lost Foles and the Cardinals lost Palmer and Stanton Seattle was in danger of missing the playoffs altogether, and ended up only 2 games ahead of the Eagles who were the first team to miss the playoffs from the NFC and finished tied with the Packers and Cowboys for first. The gap between Seattle and the other playoff teams isn't as large as it appears, and the draft and free agency will go a long way to determine how big it really is. Seattle is going to run into the same problem the 49ers had when they were on top of the world, which is money. The NFC should be pretty fun next year.