(Not sure if you're being sarcastic or not…)
Xerneas, Yveltal, Zygarde
X, Y, Z
We are 100% certain that Zygarde completes the legendary trio. Plus, Diancie is an event pokemon and Zygarde is catchable in game. So, no, Diancie would not be on the cover.

the 2DS is more of a original GBA since it's less compact, no backlight and an allover downgrade
-the 2DS also isn't available in japan since it's made for US/euro soccer mums who are afraid of 3D destroying their children's brains
-also Z isn't a chromosome,
-the same also could've applied to DPP and HG/SS but it didn't, platinum was released prior to HG/SS, if it would've applied to those too that theory would be a bit more believable.
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besides, instead of a Z edtition, i'd wish gamefreak would make some good DLC for XY, since there's the possibility to do so on the 3DS and other games already got some quality DLC