If they released a new external display, it would have to be retina quality. That makes me think that Apple isn't yet confident in MacBook/MacBook Pro GPUs to drive multiple external retina quality screens like you can do now with the Thunderbolt displays. Maybe the top-of-the-line machines with the Nvidia/Radeon GPUs, but it doesn't seem very Apple like for them to release a new display with a disclaimer: only works with Nvidia/Radeon GPU equipped Macs.

Retina quality 27" displays need to wait for the next Thunderbolt or similar speed bump. The display requires > 20Gbps to feed. That means waiting for Intel to pop out ThunderBolt 3 with the newly announced DisplayPort 1.3 38Gbps connection.

Too many pixels, too little time.

As far as GPU processing power, I wonder how much more GPU it takes for two displays. I use two, but I don't do heavy updates on both at once. I suppose you could imagine some video game wanting to use both, but for professional work it seems mostly about real estate, peripheral awareness, and context switching with a head turn. It probably wants more RAM for compositing, but that seems ample already.