Even prior to Alex's diagnosis, my wife and I were discussing this a few years ago; namely, who could, and who should, replace Alex? She wants Anderson Cooper, and that was one I had thought of as well as he has hosting experience with The Mole and would be well-read like Alex was (though, as someone already mentioned, politics might keep him out of the role). My initial vote went, actually, to George Stephanopoulos or to Dan Harris, both for the same reason: well-read, similar demeanor to Alex, would not dominate the proceedings. I'd prefer Harris, mainly because I thought he did a great job on 500 Questions, and I don't know if Stephanopoulos would give up either GMA or This Week. I love Tom Bergeron, but I personally don't see him as a J! host except maybe for a celebrity edition.
My money at this point: KenJen comes in as interim to finish up the season, and then he is considered along with other candidates for the permanent position over the summer break when they have more time to vet. I don't see Richards doing anything other than substitute hosting, as he just started as producer this year. If they don't want to do half a season with a new host, then they can just do a "Best of Alex" set of episodes to fill out the season.
All pure speculation at this point, of course, and I'm sure we'll have more answers in the coming weeks. For right now, I'll enjoy watching the master at work.
Anthony