I doubt we'll ever see it in game at this stage, but I do like @Stratago's theory, and I totally agree with him. Zekrom and Reshiram were essentially born out of the sheer power of the original dragon, and Kyurem was pretty much the carcass, we're told that in game. I would really love to see this later on; perhaps in a few Generations time they'll release they Grey Version that we had originally anticipated and this will be the plot line, or at least a sub-plot.
A few problems are raised though. For example, what would its typing be? Would it remain Dragon/Ice and get a larger move pool to include Electric and Fire attacks? I don't think it would become Dragon/Ice/Fire/Electric. People have speculated about triple typings (I personally think they won't be added; they'd break the game mechanics), but I think we can all safely say that they definitely wouldn't have Quadruple typings. Dragon/Psychic maybe? And what event would have to occur in order for this to be possible?
Also, @Stratago, that drawing is awesome. If we ever do see the original dragon, I really hope it looks something like that. Nice work.