Very interesting set. Mixed Sweeper Altaria is really overlooked.
Anyways, I'll be making a few suggestions:
The EV spread should look something like this: 248 HP / 168 Sp. Atk / 92 Speed. (stolen from Smogon without any regrets :P).
>The given EV spread prioritizes bulk, which is crucial for Mega Altaria to handle threats such as Mega Lopunny and Mega Charizard X, and special attacking power. The Speed EVs allow Altaria to outrun Jolly Azumarill; it is generally advised to invest a bit more in order to outspeed defensive Rotom-W and Landorus-T, which commonly run enough Speed to hit this benchmark. Depending on the team, more Speed can be run; some benchmarks include 240 for Adamant Bisharp (176 EVs) and even max Speed if outrunning Adamant Diggersby and slower base 100s is desired. Another option is 252 SpA / 4 Def / 252 Spe with a Timid nature, sacrificing bulk to outspeed certain key threats, including Jolly Mega Heracross, Jolly Bisharp, Adamant Excadrill, Adamant Lucario, and Timid Heatran, as well as tie with opposing base 80 Speed Pokemon such as opposing Altaria, Mamoswine, and non-Mega Gallade.
A really good explanation about the EVs above. Moving on to the moves...
Now, your moveset is pretty good, but I'll make only make one small adjustment.
If you're thinking about replacing D-Dance, then I would suggest Heal Bell (for team support) or Roost (for a reliable recovery). I'm not sure if you're willing to put in effort to get these moves on Altaria, but it'll be worth it.
That's it. I hoped I helped!
Source: Smogon - Altaria, Experience