How would you…in glorious detail…imagine kyurem, zekrom, and reshiram finally combined?
(Disclaimer first: I’m not a designer or an artist, and a Google image search would give you multiple answers to this question that are better than anything you’ll get from me)
The thing is, I actually like that Game Freak never gave us a final realisation of this concept. Whatever they came up with, it would not have lived up to our expectations or done justice to the idea. The original primordial dragon represents the totality of all truths and the realisation of all ideals, the reconciliation of every pair of opposites and the resolution of every conflict. I suggest, though I obviously cannot prove, that the reason it never appears in the games is because Game Freak realised that there is no satisfying way to depict that, and decided it was better left as a mysterious background presence in the lore. Sometimes it’s more effective to leave things to the imagination; there’s a reason some horror movies never show the monster. A big mass of black and white wings and scales and $#!t is not as evocative or meaningful as the vague suggestion, buried in layers of mythology, of a primordial being who symbolises the impossible unity of all divisions. Frankly I think Pokémon could do with more of that kind of restraint, not less.
So my real, honest answer here is one of my classic troll lines when faced with a “how would you…?” question: I wouldn’t.
You can go here for some earlier thoughts on the basic notion of how we might expect to see the original dragon restored. But as to actually designing it…
The problem is that Reshiram and Zekrom are intentionally designed as opposites – Reshiram is bright, feminine, avian, graceful, intellectual; Zekrom is dark, masculine, saurian, forceful, passionate – and the whole point of yin and yang as a concept is that all opposites are part of each other. Any restored and reunited form has to have all of those characteristics at once, as immediately obvious and powerful as they are in the dragons we know, combined but undiminished. While doing all of that, it also has to avoid the chimeric appearance of Black Kyurem and White Kyurem. Those are unnatural and incomplete forms created by empowering Kyurem with the unbalanced half of an unsolved equation. What we’re trying to create is supposed to be the restoration of what these three were before they split; it should appear natural and whole, as if it had been designed first all along, and the other three were somehow distilled out of it. And that’s… an impossible design challenge. I’ve seen a lot of fanart that attempts to illustrate the dragon, all of it strikes me as, at best, underwhelming (a lot of examples look more like fusions of Black Kyurem and White Kyurem than fusions of Reshiram, Kyurem and Zekrom), and I unequivocally cannot do any better. When I try to think of an appropriate design I just come up with, essentially, a ball of light, and even that is not only unsatisfying but clearly wrong, because it would need to be dark at the same time.
The best I’ve got is to have it literally be Reshiram and Zekrom. When reunited, they become forms of each other, and can just be one or the other at will, shifting between forms in a flurry of white feathers and black scales (maybe with some visible permanent aura that lets the opponent know they aren’t just facing Reshiram or Zekrom alone). They retain two separate movesets and EV spreads, except that both have to learn new signature moves: a physical Fire attack that transforms Zekrom into Reshiram (White Flame?), and a special Electric attack that transforms Reshiram into Zekrom (Black Lightning?). Reshiram and Zekrom are both facets of what this dragon is, but the whole is beyond comprehension, and we can perceive only one at a time. And I’m not happy with this either, because it leaves out any role for Kyurem other than as an intermediary in the fusion process. Thematically Kyurem represents nothingness or absence, and in the lore it’s supposed to be the shattered remnants of the original dragon’s body, and a vessel for the truths and ideals embodied by Reshiram and Zekrom. This could mean that Kyurem is an anomaly that will resolve itself out of existence altogether when Reshiram and Zekrom are reunited; or it could mean that Kyurem is the crucial lynchpin that allows them to exist as one, and should actually be more important in the fusion than either. I almost like the idea of Kyurem emerging from Reshiram and Zekrom with a fresh HP bar when they are defeated, and being able to “revive” itself back into Reshiram and Zekrom if it keeps its health high over several turns (overpowered? Absolutely, but when you’re looking to turn Reshiram and Zekrom into something more ancient and powerful still, you’ve left game balance behind long ago). Then it starts to feel clumsy or tacked-on, though.
No one around here ever gets what they ask for, you understand. It’s fair though, you see, because I don’t either.