Maybe it's me not being able to properly understand what you said here (i'm not an english native speaker), but Salamence and Garchomp weren't balancers of anything. Salamence was banned in Gen IV and I still remember the communicate from Smogon, its unpredictability and wide variety of sets -essentialy DD or MixMence- were too much to handle. And Garchomp was banned in its own generation (Gen IV), not only due to its typing and stats, but because its only ability was sand veil and you couldn't destroy team building banning Pokémon like T-tar, whose only ability was Sand Stream back then.
I don't see why I should get excited or how this reponds to the unpredictable effects of a meta we don't know anything about.
It's a simple oversize of the mon with more HP. The end. And a pool of unique moves, one for each type. Comparing this to a fully new design, with total BST changes, retaining of the Pokémon movepool while transformed, potential changes on their typings are abilities, shouldn't be discussed. In fact, there's no place for comparison.
I don't know if competitive is the only thing you care about, as this topic has been adressed before, but not only depics a sincere, complete lack of effort from Game Freak's part, one that i just objetively described, but this is less atractive in both senses, competitively and in regards to in-game content.
But again, if you think I should be equally interested in seeing a potencial Stantler evolution with new desing, new ability, maybe even new typing, and to see how all this changes can affect it in-game and competitively, as to just seeing the same Pokémon with size==6, sure. Sure it's a great, hard-worked mechanic.
C'mon, let's be serious for a moment and look at the obvious facts. Let's not lose to complacency.
I'll ignore the example you provide because of two reasons. One, it is theorymon and should be forbidden. Secondly, because an infinite number of similar examples and be depicted using Mega-Evolution. In fact, just due to te changes in BSpeed, Ability and Typing, anyone can do a far, far greater example of an ideal an hypothetical scenario in which all this elements combined turned a battle. There's no point. Just get the general idea. Oversizing a Pokémon and giving it 50% plus HP while transforming all its moves in unique attacks would never be more interesting than changing the Pokémon design, giving it a new BST distribution, a new typing or a new ability, or both. And less factor will be involucrated. Simple as that.