A lot of the candidates being floated here are well into their 50s-60s. If they're gonna choose anybody, it has to be someone in their 30s-40s that they know will want to stick around as long as Alex did.
In reality, I could totally see them filling the rest of the season with guest hosts -- Jennings, the Clue Crew, hell get Pat Sajak for a week if you want -- and then having a definitive choice before next season starts. Either through whoever guest hosted, or pull a Price and get someone out-of-left-field but has name recognition. Maybe this is Neil Patrick Harris' time to shine?
Regardless, I wouldn't mind seeing Stephanopolous give it a try, but I'd also see some of what y'all suggested as well like Bergeron, Walberg or Kiernan.