At this point, there are three cards leftover. It seems fairly logical that that means three more tankoubons, one for each card. However, popular theory is there's another possibility: two more tankoubons and one more artbook to close the cycle and be the match for X/Zero.
19. The Sun - ?
20. Judgement - ?
21. The World - ?
As for who will be on each... well, obviously Fuuma will be on one of them.
My original theory, and the one I still think is the more fitting, is that Fuuma will be Judgement. The card's meaning fits perfectly, AND it would close the cycle, if there are only 20 tanks. It would end and start with a Kamui, in other words.
However it's also quite possible that he might be The Sun. That really wouldn't have occurred to me, except each tank's spine has a "hint" of the character coming in the next one, it's pretty clear that X18 is heralding Fuuma's coming (you can see bits of his hair). I still think he migt turn out to be Judgement though, and that CLAMP is throwing us a red herring (there COULD be someone else on X19, with the side of Fuuma's face as background).
But okay, The Sun could also work for him, rather well. In fact, I'd snicker, a lot, if it was him, because it sort of falls right inline with a lot of my theories about Fuuma. But I digress.
Taking Fuuma into account still leaves us with two faceless cards... and several possible candidates. I've sort of narrowed /them/ down to Hokuto, Keiichi and Nokoru, simply because they've all played somewhat important roles. Daisuke might also be one, but I don't know, I don't find a plausible explanation for him to be. As for who will get each card, I'm kind of partial to the idea of Nokoru (or Nokuro, Suoh and Akira) getting The World. Among the others, if Fuuma is Judgement, I would say either Keiichi or Hokuto (most likely Hokuto) could both be The Sun. It doesn't really fit Daisuke though. If Fuuma is The Sun, then I have absolutely no guesses who will get Judgement instead. ^^;;