I highly doubt that there will be a 3072 CUDA core 980 Ti. Titan X, unlike the original Titan, is not compute optimised and has the same FF64 limits of all other GeForce cards. Essentially if they release a 3072 core 980 Ti, it's only difference with the Titan X will be lower VRAM. That's not enough of a differentiator for two distinct models with big price difference. What about SLI? If the core is the same and all that's different is the amount of VRAM, they should even be able to SLI. CANT HAPPEN. NO WAY.
I would say reducing the VRAM to 6gb is quite a difference and very likely the way NVidia will go.
My guess is that the 980 Ti will be in the range of ~£600, has around 2500 CUDA cores, has 6GB, is 256-bit, and is slightly higher clocked than the Titan X so Titan X is roughly 20% faster and maybe 30% more expensive. When it's released, 980/970 will get a little cheaper.
Not very likely as it would mean NVidia having to produce an entirely new GPU core.