A door could be a 4-6 custom screw like the bottom of the laptops. (although laptops have more screws. But the door only needs to be only so big. ).
Similarly a design where the whole back screws off (again like the laptop's bottom). For some reason, Apple has skewed the current iMac so that the screen goes in last in assembly. That is one of the primary root causes of the problem.
A very long time because both expensive (and TB Display are already relatively expensive 'monitors' ). Additionally for the lack of GPU horsepower in primary targeted Macs: laptops. The TB Display is a mismatch for an iMac and an even bigger one for the Mac Pro. The dangling power cord does nothing is highly indicative of that.
The GPUs in the laptops are as weak or worse than the iMacs.
TB isn't going anywhere for another 2 years. (that's just to a new standard. They will be another "v2.0" product roll out associated with that which will have a "build to volume" ramp up. )
http://www.anandtech.com/show/5405/the-first-thunderbolt-speed-bump-likely-in-2014
TB doesn't support DisplayPort 1.2.
TB is somewhat unlikely to snag PCI-e v3.0 in the first upgrade. To be a reasonable v3.0 conduit it would need to be faster than v3.0.
At best you may see some controllers go from x4 v2.0 connections to x2 3.0 connections so that they don't consume so many PCI-e lanes. But that gives you a net zero speed increase. There is little reason to do that as most likely will only disappoint more than please. Also because the TB controller is also a PCI-e switch. having 4 1x PCI-e v2.0 links coming out works great for docking stations suppling 4 kinds of external sockets.
As long as Intel's reference designs hook TB to the IOHUB support chip which has just v2.0 lanes the TB controller will probably stick with v2.0.