What if Smaug the dragon lived?

[Thingol]

"Yet things might have gone far otherwise and far worse. When you think of the great Battle of the Pelennor, do not forget the battles in Dale and the valor of Durin's folk. Think of what might have been. Dragon-fire and savage swords in Eriador, night in Rivendell. There might be no queen in Gondor. We might now hope to return from victory here only to ruin and ash. But that has been averted because I met Thorin Oakenshield one evening on the edge of spring in Bree. A chance-meeting as we say in Middle Earth"

The Return of the King, Appendix A

If Sauron had regained the ring he probably would have gained the ability to control Smaug and any other of the evil entities from the 1st age, at least to some degree. Gandalf obviously thought that Sauron would have used Smaug to attack Eriador and destroy Rivendell. I tend to agree with Gandalf, Smaug would have been a terrible weapon against the last opposition to Sauron. He would have burned the forests and destroyed any resistance.