My money is on Beldum being in the next update. Statwise, he's not the STRONGEST pseudo-legendary, but he's pretty damn good. He also doesn't have a wide variety of powerful moves, the main ones being Meteor Mash, Hammer Arm, and Zen Headbutt (psychic doesn't count on account of his average Sp. Att. stat). He does learn Hyper Beam later on, but that won't be controllable until, like, Episode 11 (though, Meteor Mash is probably still stronger if you calculate STAB in).
EDIT: NVM, it's probably gonna be Hydreigon's line, as that pokemon has the most weaknesses next to Tyranitar, and the most average stats of the legendaries. Definitely not gonna be the Salamence line, as that one learns Fly on its own, and Ame wanted to keep that one for later down the line. No Dratinis because Ame already said so. I realized Metagross is a freaking TANK, so he's likely out, though he's my favorite <3. We MIGHT see Tyranitar as it has the most weaknesses, but that Sandstorm ability might throw that out. Garchomp is pretty fast, and hits heavy, and has a decent move-pool, so he's likely out. Which leave's Deino as the best suited pseudo-legendary pokemon's starting form to introduce.
EDIT 2: Hydreigon has the highest Sp. Att. of the pseudo-legendaries, so it's more likely Tyranitar, as there are already pokemon that change the weather automatically available in-game.