Unlikely to be any major step up from the 840 Pro, SATA6 is and has been a major bottleneck for a while now. The biggest development we're likey to see in traditional 2.5" SATA6 SSDs would be capacity expansion and cost effectiveness, something we saw with Samsung's EVO and now Crucial's MX100 (~250GB drives might finally drop below $100, and we might see up to 2TB drives to become relatively common, at least as much so as 1TB is now)
Now while replacing an 840Pro drive with one that is 2-4x the capacity but no real performance improvement could easily be considered an upgrade, if you're looking for significant increased performance you're going to need to look beyond SATA6.
M.2 appears to be the option with the most steam behind it
right now, even though SATA Express is supposed to be the true successor, as much of a hack-job as it is (which is probably why there isn't that much buzz behind it). But he biggest potential comes from straight-up PCI-e drives, an area Intel just kicked down the front door of:
http://anandtech.com/show/8104/intel-ssd-dc-p3700-review-the-pcie-ssd-transition-begins-with-nvme
M.2 might fade into obscurity, reserved only for SFF systems and laptops, etc, while desktops use a PCI-e drive supplemented by SATA drives in some form (would we even need SATA Express for our supplemental drives?), kind of going back to that model where we once had a single SSD as our fast boot drive with HDDs to back up as storage, only now it seems likely we will see a PCI-e drive as our boot drive with high capacity SATA SSDs as our storage
