In my opinion, a third version would make the plot of BW a bit akward. Lemme explain: in the og versions 3rd versions that we have, the villain goes after the box art legendary; Team Magma/Aqua go after Groudon/Kyogre while Cyrus goes after Dialga/Palkia (I've played HGSS but not Crystal, so I can't really speak for that one). In the 3rd versions, the plot is almost the exact same, but the baddies go after both legendary Pokémon with some kind of interference from the third of the Trio, Rayquaza/Giratina.
In BW however, the plot is heavily engrained in this Yin/Yang dynamic. You, as one of the legendary two heroes, must fight with Zekrom/Reshiram against the other hero, N.
Now, if Ghetsis did a Cyrus in the og BW, that would alter the plot of the games so heavily that the entire premise would have to be re-written. None of the key Ying-Yang elements would be kept if Ghetsis took both dragons, and you might as well just write something else entirely at that point, which is what they did, and I'm so thankful for that.
If you think about it, BW2 do kind of follow the "3rd version formula". The region stays the same, there are new features, and the baddie goes after the third Legendary Pokémon. But BW2 were SO much more interesting than mere 3rd versions. It's such a shame they couldn't do the same with USUM.