User blog comment:Xilinoc/Ch. 610 - Mausoleum of Skulls/@comment-25891035-20141218194655/@comment-5693502-20141218201417

Of the five we know three of them. Minus Kenpachi, who I'm more than sure has nothing left to offer, we're left with two known SWPs. Minus Aizen, who is still stuck in Muken (for now), we're left with one known SWP. Oh look, it's Ichigo...

I'll make an educated guess and say that Uryū is another SWP (someone that Yhwach successfully recruited) and the last spot is anyone's guess as far as I'm concerned...

So as it looks right now, in my opinion anyway, Yhwach is only really threatened by Ichigo and another unknown SWP (since Uryū is under the thumb).