Possible, but I imagine it would be very difficult to pull off since Ultear is wanted dead or alive for her actions with Jellal.
Also, I remembered some other clue on the matter of Rogue and Sting being "old style" Dragon Slayers that were raised by dragons. Sting called Rogue Gajeel's apprentice. IMO this is yet another hint that they are just "new style"/artificial Dragon Slayers. If the dragons had returned to raise new Slayers, they'd probably have chosen "fresh" kids.
But even apart from that fact, it's still up to debate in the first place, if Natsu and the others are really just human kids that were adopted. Remember the festival arc, where Natsu and Gajeel couldn't pass the runes and how Zeref seems to know Natsu... wouldn't surprise me if they aren't even human at all.
So, the only thing that could work, would be that these two had been raised by other, yet unknown dragons.
Last edited by Loke; 11-08-2011 at 01:02 PM.