I feel like Alpha Flight is no more niche-y than New Warriors, and we got an Omnibus of that. Sure, there was maybe a new New Warriors series on the way at the time, but it's not like that was going to move so many more units to sudden new fans.
That said - with Alpha Flight, we already got three Classics deep. I'd love it if they just tacked on an Epic-sized TPB after that to get us deeper into the run.
As for Secret Avengers, as discussed above - I think it was one of Marvel's sleeper hit series of the past half decade and I don't understand how people are not crazy for it. The first run was solid, with Ellis's issues being the best one-off Avengers stories in years. Spencer's run was a modern classic for me, and Kot was utter pandemonium and perfection. I'd be first in line for any of those OHCs or Omnis.