The dragon could have died from any number of things, not the PC's necessarily, as Christopher suggested, so you can avoid having the PCs be uber-powerful at the start. Alternatively, maybe they had to sacrifice some of their power to be able to defeat the dragon in the first place (like Ged closing the magic rift in Ursula LeGuin's book The Farthest Shore). That could lead to an interesting situation where everyone expects great things of the PCs but they really aren't that powerful anymore. That said, you would really have to explain things to your players ahead of time, because most player hate it when their character becomes weaker.