Well it is different for Joker then for the Hoppers or Zabee. Joker is Shotaro on his own, so Double wouldn't work without him. We don't even know yet if the hoppers will be able to fight their Zabee counterparts yet. I do understand your desire for a dream match, so I hope ya get it.
^this, W cannot work without shotaro being in, so joker being a single entry out of the question, unless if you use joker, the body side of W becomes terui ryuu(well, it was hinted in the series that he's more suitable to be W's "body" than shotaro anyways, by W's creator)
and don't bring out the "but BLACK and RX were one guy, but they gave him a different spot for each anyways" reason, that's just because:
1.there no such thing as too much Kurata,
2.BLACK and RX represents 2 different series, not just riders,
3.like den O, Kotarou already officially teams up with HIMSELF a few times over, and not just in Decade, Joker has none of those magical moments to justify the special spot (yet).
mirror matching were always almost impossible in CH except for the guys covered by decade form changes, but i hope they separate birth&proto now, since even in RG2 they're separate and the only reason they didn't in CHF is because BP doesn't have a finisher yet back then