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.
Who is going to watch a show they already know the ending to? CBS would want a show to last for years and years... if you already know who she is going to end up with, and in fact HOW she meets him... what's the point? It seems like a small number of people would be interested in such pot hole filling story telling.
People are struggling to stick with THIS series...