I still find it odd, how Eternal is playable, instead of Skull.

nothing new on Twitter today, just the bat grongi, now with extra Kuuga on the pic

That seems like a bit of an overstatement. If the tag partners were full characters, they'd just be selectable.
The picture in question:draewon translated it properly, no overstatement here. However, I believe they mean that you can change your Tag Partner anytime during missions to suit your playstyle.
i love how the Twitter update guy likes to state the obvious, of course we can team up with non series Riderps, how else would the bulk of the showa Riderps have partners if only same series Riderps are allowed to team up....not to mention the trailers already showed us how possible this were"Today's screenshot is Kamen Rider Agito fighting alongside G3-X. Even though you can team up from Riders from the same series, you can also team up with other Riders as well"
i love how the Twitter update guy likes to state the obvious, of course we can team up with non series Riderps, how else would the bulk of the showa Riderps have partners if only same series Riderps are allowed to team up....not to mention the trailers already showed us how possible this were
speaking of which, it's a crime that they didn't have Tackle as NPC partner for Stronger...