I've always used Flash Cannon on my magnezones...huh.
Actually I think Magnezone is one of the very few Steel-types that isn't crazy if it decides to run Flash Cannon. But now we'll see stuff like Empoleon, Dialga, and perhaps even special Lucario use it. Heck, even the Fairy-fearful Hydreigon might start packing that Flash Cannon to punish Fairy-type switch-ins.
If the whole Steel thing is true, Dialga will become the only Dragon with a good Fairy defense. Unless they give us a Poison/Dragon...
Actually, it would still take SE damage from Fairy-type moves, since Steel wouldn't resist Fairy, but the SE hit on Fairy with Steel-STAB would help considerably. Defensively, it's actually Lati@s and Reshiram who would be the only Dragon-types to take neutral damage from Fairy.