It would be nice for every team to have a rivaly game each season. It couldn't happen until after the current schedule rotation is over in around 6 years, but it would be great for the areas where "cross town" rivals exist.
Giants/Jets
49ers/Raiders
Texans/Cowboys
Ravens/Redskins
The problem would be with teams that really don't have a natural rival, their rival is already in the division or there's more than one "natural" rival (Tampa Bay, Miami and Jacksonville).
BTW: I predict that

will be the most used smilie.