No one can really say until we know where the 100 new base points get added. I think many are hoping it gets a more defensive spread due to two things. 1) it's many impressive resistances 2) that fact that even if it's upgrades are primarily offensive it'll probably just end up being outclassed by other offensive Dragons and Megas. (we already have to Dragon Danceing Mega Dragons who will be dealing more damage even if they dump more than 50 of the 100 into Alteria's atk)
But assuming it gets a mostly defensive stat increase, Roost will be a staple and then you want some combination of support and one (or maybe 2) attacks. Some obvious picks for support are Heal bell, Perish Song, Sub, and Toxic then some other more niche things like Tailwind, Cotton Guard and Roar/Haze (which have some possible utility over Parish Song but all 3 have the same general affect). What attacks it should use will totally depend which offense gets the bigger boost or if it's essentially even. But attacks to keep your eye on are the best variants of each STAB so Hyper Voice/Dragon Pulse or Return/Dragon Claw. And if it gets a mostly offensive increases then you can consider possible Dragon Dance sets (that will likely be outclassed) and coverage moves like Earthquake (physical) or Flamethrower/Fire Blast (special) for coverage on Steels that resit you dual STAB. Alteria has a fairly reasonable Special move pool in addition to things I already listed. But until we know what it's mega stats look like we can't really say for certain what sets will be best.
It's all theorymon, but I wouldn't consider an offensive DDing Altaria necessarily outclassed by the current crop of DDers. It hardly lacks for power, even with a likely lower raw attack stat, when you consider it has access to STAB Pixilate Double Edge, and STAB Pixilate Facade to mitigate any attempts to burn it. Even if we assume Altaria's only going to gain +20 attack and special attack:
252+ Atk Pixilate Altaria Double-Edge vs. 252 HP / 252+ Def Unaware Clefable: 192-226 (48.7 - 57.3%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252+ Atk Pixilate Altaria Double-Edge vs. 4 HP / 0 Def Bisharp: 217-256 (79.7 - 94.1%) -- 6.3% chance to OHKO after Stealth Rock
252+ Atk Pixilate Altaria Double-Edge vs. 4 HP / 0 Def Mega Tyranitar: 306-362 (89.4 - 105.8%) -- guaranteed OHKO after Stealth Rock
+1 252+ Atk Pixilate Altaria Double-Edge vs. 252 HP / 252+ Def Hippowdon: 210-247 (50 - 58.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+1 252+ Atk Pixilate Altaria Double-Edge vs. 252 HP / 0 Def Rotom-W: 307-363 (100.9 - 119.4%) -- guaranteed OHKO
+1 252+ Atk Pixilate Altaria Double-Edge vs. 252 HP / 0 Def Azumarill: 391-462 (96.7 - 114.3%) -- guaranteed OHKO after Stealth Rock
+1 252+ Atk Pixilate Altaria Double-Edge vs. 4 HP / 0 Def Multiscale Dragonite: 340-402 (104.9 - 124%) -- guaranteed OHKO
Couple that with rocks neutrality, a fantastic defensive typing to make setting up easy, and access to Sing for counters, and the fact that fairy is a supremely spammable type, and I wouldn't say outclassed. And that's assume Altaria only gets +20 attack - an amount which would likely make it extremely bulky.