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Alright here's a compilation of Khu's newer tweets on these new form concepts to try and make some sense out of some things. Here's what I understand:
Past/future forms
>this is what Khu has been referring to as "new concept" (not RFakes)
>Jigglypuff/Amoonguss leak
>there are a lot of them, possibly around the amount of megas in XY
>are "OP" mons in battle in the same vein as megas
RFakes
>not past/future forms, but are also new Pokemon that look like regional forms
>eg. the Diglett/Dugtrio "leak" is a new eel Pokemon that looks like them
>not many of these compared to past/future forms
Both styles of new forms are new Pokemon that take up dex numbers.