I don't think the 780Ti will be anything new or amazing, there already isn't much room between a Titan and 780 so performance wise it will be nothing new. What would be more interesting is if it takes the 780's pricepoint and bumps everything down a price tier, that would be a good move to combat the 290X.
Of course there is also the speculation that the 780Ti is a completely new card and architecture, essentially being a Maxwell card released a generation early. I would guess in the Maxwell lineup it would be the equivalent of a GTX 870 or 860ti. Its not unheard of to do this, AMD did it with the 7790, exact same as the 260X except for a VRAM upgrade.
If I were either side, I would start working on more features like Mantle, PhysX, G-Sync, TrueAudio and the like. Current top end cards at 1080p, there isn't any difference, all will run your games at 60fps. Until 4K becomes viable for the average gamer, the performance crown becomes irrelevant until we have the monitor to use its full potential, so until then, more features or maybe AMD/Nvidia could subsidize the cost of such displays to hasten its arrival to the desktop.
Whatever happens, it can only be good for us.