It wouldn't be that impractical. It's base speed (from memory) is something like 60, so after one DD it would only be tieing with base 90 pokemon. Still due to it's numerous weaknesses, a typically counter or a choice user would beat it.A Dragon Dance set with Aggron would be deathly, specially with its high defenses. The thing that I'm hoping for the most is a Rock type attacking move with a good secondary effect. I could totally see a Rock type move with a Paralyzing secondary effect, something similar to Body Slam where its paralyzed from brute force.
Good point with the Keldeo forme...they did make it seem like it'll be more than it really was...hopefully they'll make Resolution Keldeo stronger this generation...And based on the word of the creator, Sky form Shaymin, Black Kyurem, White Kyurem, and Resolute Keldeo are all "new Pokemon" and not Pokemon forms. While were on it Keldeo's Resolute Form is "Supposed to be stronger than his original form" there was no stat change with it. They hyped it up as his stronger form for both the movies and when he was released for B2W2. They can still be stronger even with a debuff, change in type for Ampharos and different abilities like Mawile's Huge Power makes them stronger already. Having different stats makes them stronger as well, but they'd still get debuffed to balance out the changes.
I'll be honest: I'm not personally fond of using DD on Pokemon that have less than like, 80 speed. One has to realize that Aggron has base 50 speed, so 1 DD would bring it to about 75 base speed, and another to bring it around the realm of 112. Which isn't too shabby, so to speak, except...who's going to let it set up that much? Having a weakness to the most common attacks in the game (and no way to really compensate for said weaknesses), it seems that Aggron is doomed where it is.
If it were to get a mega-evolution, then I'd hope that it would sacrifice either it's Steel or Rock typing for something more practical (though I'd prefer it to keep it's Rock typing if its type does change, for the sake of the 50% spdef boosts in Sand).
Here's the original source to the rumor: https://pastebin.com/C6761nicFor some reason, I'm not really seeing this dex leak that everyone is talking about, even when I search on google. Would anyone care to provide a link, please? D:
Anyone can make a list using the Pokémon already mentioned in more reliable rumors...so there's no added credibility in my eyes...