I've noticed something. It seems that the higher level the zanpakuto, the more control over something in its bankai. This translates to many manga/animes...
For example, in JoJo, you initially had Stands that basically controlled little things. The protagonists eventually increased the power of their stands to a point of nearly god-like ability to fight the last baddie.
Even as recently as a couple of months and weeks ago, we saw Izanagi and the Moon's Eye in Naruto.
In Bleach, will we see the final baddie with a God Zanpakuto, with a bankai to control everything?
Here's my prediction on this. When the Hougyoku made Aizen's zanpakutou disappear, it didn't just turn into nothingness, the Hougyoku absorbed it.
As others have stated, the Hougyoku will probably end up manifesting itself into a humanoid form, and what do I think its weapon of choice will be? That's right, Kyouka Suigetsu. This is when we'll see what would of been Aizen's bankai.
If im not wrong it was stated that ryuujin jakka was the strongest of all zanpaktou. If there was a stronger zan than ryujin jakka it might probably belong to a heavenly being .. or a hellish being with a devilish zan ?