Floyd Mayweather hasn't had a truly close fight in over a decade and Andre Ward has never had one. So it might be a bit of an exaggeration to argue that there is a "nightmare" fight out there for either man.
But there are certainly opponents available who would make for compelling matchups with both undefeated champions.
Sugar Ray Robinson was the greatest pound-for-pound boxer of all time, but Jake LaMotta was consistently able to push him to his limit. Muhammad Ali may truly have been "The Greatest," as he claimed, but Ken Norton and Joe Frazier were both able to beat him once and give him close to all he could handle in rematches.
No fighter is invincible. Not even the all-time greats.