Is Dromoka, the Eternal constructed playable? Not sure. It seems like it was pushed to be, but there’s other cards competing for its spot (Wingmate Roc, Nissa Worldwaker and Ajani, Mentor of Heroes all come to mind).
However, the cycle of legendary dragons has clearly been pushed for constructed.
Dromoka does have its benefits. Its potentially bigger than Roc (which is 6/8 in two bodies) and it has the added benefit of being a dragon in a format where it might matter (Crux of Fate). The addition of Valorous Stance might also be a big thing. Being able to protect the dragon might be gamebreaking and you can probably deal with most of the early creatures from your opponents with your cheap, quality creatures.
Things like Elvish Mystic might be a good addition. This deck is (as it stands) relying way too heavily on being able to overcome one-on-one removal. It does have enough creatures, but that doesn’t help much against sweepers. With the Mystic you might be able to get your bigger creatures out before your opponent can draw the correct removal. On the other hand, currently allied colored aggressive decks are in a bit of a bad place, since you don’t have a land that would let you easily play cards of different colors on curve. For example, in this deck, there’s no way you could cast a Brimaz on turn two with the Mystic, whereas it was possible with Temple Garden.
This strategy might still work. Just plenty of quality creatures attacking from turn two onwards. Its not a bad strategy. Just might not be good enough.