Well if thought about logically a new TBD would only come out along with a computer announcement of some sort .... The best hope is it will get announced when the VERY overdue MacPro gets its day in the sun.
Except, if you look at the publicity for the ACD and TBD, they're very much being pitched as second monitors and docking stations for MacBooks.
I suspect a lot of Mac Pro customers will also go for specialist, third party monitors (the TBD is more 'prosumer') or different size combinations (I'd seriously consider 2 x 24" rather than 1 x 27" to go with a Mac Pro). There's also the question of whether a new Mac Pro would even support Thunderbolt as a display connector: a major selling point of the Mac Pro is the ability to take specialist graphics cards - they'd need some way of feeding their DisplayPort signals back to the motherboard so they could be fed through Thunderbolt.
I've heard conflicting rumors that Thunderbolt allows graphics cards to go into display. So in theory someone with say, a 13" Retina Macbook Pro would get a performance boost when connected to the display.
AFAIK, yes, they could put the 'graphics card' in a Thunderbolt monitor - Thunderbolt is, after all, basically 'external PCIe'. The problem is that Thunderbolt only supports 2 PCIe lanes per port whereas high-end graphics cards prefer to sit in the 8x or 16x PCIe slot in a PC - they'd probably work on Thunderbolt, but with limited performance. Since even the MacBooks with Intel integrated graphics
can drive an external monitor well enough for 2D use, its hard to see what the market is.