Spin-off wouldn't work so well, considering one of the main themes of this show is that Ted is telling a story of his youth in the year 2030, but that his story is contemporary with us, the viewer. A spin-off wouldn't be able to use that same theme, and instead would have to go "The Wonder Years" route.
Mother isn't exactly tied to a specific period of time, i.e. it's just a modern age sitcom and it wouldn't be much of a hassle to set a show about 10 years ago to keep it in continuity with the original show,
It could make sometimes references to events in the original show and maybe even show us legen.. wait for it.. dary scenes that happened in Mother from a different angle telling us that the Mother was many times so close but Ted wasn't yet ready.
Heck.. there were several scenes where Ted nearly met the mother but missed her by seconds or the time where Ted first teaches (the wrong) class and it's revealed that the mother was a member of this class.
Taking this gag too far is not a good idea of course but it would be nice to throw a bone to the original show once in a while.
As with all things it depends on the writing and the cast if such a show could work.. i'd definitely check it out to see what they could come up with.