I don't agree with you those are the real issues with M.2 and U.2. It's not like there haven't been boards with three full-length M.2 sockets already and even U.2 ports have started showing up.
I wouldn't call 1TB SSDs "limited" when 2TB SSDs are unaffordable for most. The mainboard space doesn't need to be consumed if using a daughterboard, which I think is prefered for mITX boards. The 22110 format is not going to be a popular format (atleast for now) for M.2, with a 1TB SSD (without stacked NAND) fitting on the 2280 format, today.
If U.2 isn't standardized yet, why is Intel selling their SSD 750 already ? It's compatible with SAS 12Gbps backplanes too, so it's pretty much set in stone as the intended solution (U.2 port on the mainboard, SAS-port on the drive) works.