I'll be waiting for MDF to update to 40.x, but I won't play 40.x either until MDF though; Vanilla DF gets too easy, too quickly.
Humans etc does not interest me the slightest, but after playing MDF with dwarves for so long I cannot move away from all the "extras" it gives, and the difficulty is very much needed.
But I do wonder one thing about the update cycle of MDF: Won't it be easier to actually update MDF to 40.x after humans are finished, dfhack is updated and a stable branch is released instead of updating it further on the current branch? The more content, the more work after all and all those workarounds etc to get MDF into the shape it is today might not work the same in 40.x meaning that you might actually have to slice some parts of MDF away to make it work in 40.x and it'd be a shame to build it, and then have to tear it down due to the update, if you get what I mean.