It would seem to make more sense for Meredith to do it as a way to promote herself for her upcoming talk show. That being said, maybe planning for the show will leave her too busy to step into Today. Who knows.
Yeah, it seems like Couric even substituting on Today might be complicated given the partnership between Yahoo and ABC News.
On a separate note, they have a long list of internal candidates who are more than able to sit in for Savannah. Seems like it might send the wrong message to them and to some loyal viewers to bypass all of them in favor of Katie or Meredith, although if it's all about ratings, that might not be a factor in the mind of execs.