Since Kyuremm is just an empty shell, it doesn't share the OD's conscience, which
Reshiram and Zekron do share. Thus,
Kyurem needs not to be the dominant Pokémon in the fusion process. My theory is that for the OD to be restored,
Reshiram and Zekrom need to fuse naturally (instead of via a technological device such as the DNA Splicers), and then, the resulting dragon would need to fuse with, or absorb,
Kyurem. I think that unlike the other two, Kyurem can be merely absorbed because since Kyurem is just an empty shell, it doesn't share the OD's mind (i.e. it's conscience and memories); actually, given that Kyurem is only the leftovers created from the split, it might not even be needed at all; after all, we are told
several times that the Original Dragon split into Reshiram and
Zekrom, not into Reshiram, Zekrom and Kyurem; all we know is that Kyurem appeared at the same time the other two split, but not that it was part of the OD, for all we know, it might be just the scraps, and like a
Ninjask (the original
nincada Pokémon) doesn't need its
Shedinja to exist, it is possible that actually there is no need for Kyurem to fuse with the other two to restore the original dragon, like a placenta when a baby is born: the placenta is a product of the birth, but it is not part of the baby nor the mother, and none of them need it to live. Following this line of thought, Kyurem wasn't even part of the OD, it just was created as a side efect of the split process (like the placenta and the Shedinja). For all we know, Kyurem is nothing but living debris. But I'll give Kyurem the benefit of doubt and still include it in the restored OD (after all, it is the boundary Pokémon, it might be needed to 'contain' the resulting R&Z fusion), just that the dominant Pokémon must be 'Reshikrom' (that is, the resultant Reshiram and Zekrom together), not Kyurem like it was in BW2.
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