No idea personally.
AFAIK, there's:
SATA 6Gbps
SATA Express
M.2 (SATA AHCI - legacy, PCIE AHCI or NVMe)
IMO AFAIK they all suck in their present forms:
SATA 6Gbps obviously isn't very forward-thinking and we're already saturating that bus without trying too hard.
SATA Express is a ginormous connector (2 SATA-E ports where approximately six SATA ones could fit before), it's about as elegant as a bucket of bricks and seems like something that the least creative interface designer could have come up with after too many drinks in the pub.
M.2 (assuming that we're not talking about the legacy type) isn't that bad, but considering that all the solutions I've seen involve being seated parallel with the mainboard, the idea of having more than two plugged in seems absurd given the size of the M.2 boards. If they start standing perpendicular to the board then they'll need physical support in some way.
Based on my understanding of the available choices, PCIE NVMe seems like the most appropriate however it seems extremely limiting in terms of how many storage devices could be added to a given system, (assuming that my ATX board had five spare PCIE slots after the graphics card, that's not that many storage devices, and then there would be no available option to add any other form of card to the computer), so it doesn't make much sense.
I'm surprised that something like a SATA interface with more connecting pins hasn't been designed to take the throughput to 12Gbps while something wholly new is devised. Has the SATA port truly been taken as far as it can go?
Perhaps a combination of M.2 and PCIE NVMe is a way forward? A big-ass 16x card with as many M.2 connectors on as possible? I would have thought with a bit of multi-layered socket designs (like the sort that many laptops have for memory), I would have thought that one could put 8x M.2 connectors, utilising both sides of the card?