What if??

[Voralphion]

Now as for an Eagle taking on a dragon there is no competition, a dragon is much greater

Actually the eagles fought the winged dragons in the War of Wrath, so I think it is entirely possible that the eagles would be able to fight with a dragon, maybe not one on one but certainly with a group of them.

I don't think Sauron would have been able to completely control Smaug even with the ring, but they certainly would have come to an agreement and worked together, with Sauron perhaps allowing Smaug to control some land in the north, as there Smaug could have destroyed the last elven realms there such as Rivendell.