Thunderbolt Display with Inbuilt GPU??

It's certainly a possibility, but it would raise costs quite significantly as well as complexity; you'd need a bigger power supply, better cooling and so on, plus software support in the Mac itself.

A superduper GPU would be wasted on the Thunderbolt display since the connection isn't quite high-bandwidth enough to push highly complex scenes at high framerates, but a mobile-class GPU, even an upper-range model, would probably be quite well matched.

This is also the biggest issue, because GPUs age MUCH faster than the display itself does. Wait 3 years and most GPUs seem hellishly slow and outdated if you actually use them to any great extent. However, 3 years on a display often only puts it in its late teens, and can thus be expected to serve a lot longer. Imagine being joined at the hip to a lower mid-end class GPU (which is where even the best mobile chips belong) for that long! You'll go nuts in short order.

 

I can see TB-based GPU adapter. But certainly not built into the TBD itself.

 

I emailed Apple the other day requesting a Thunderbolt external GPU or at least the firmware to support it.

If that was the case, then I wouldn't need to settle for the middle-class Mac Mini. I could go with the quad-core Mac Mini that lacks the GPU and then get the Thunderbolt GPU to back it up.

Unfortunately, I haven't been able to find anything on the horizon saying such a thing is actually going to happen, just rumors.