Who they decide to appoint will only be interesting if they appoint somebody interesting, which seems unlikely. Larry Page is still running the show, and moving from an executive chairman to a non-executive chairman will probably only mean that the board has less power than they did before.

Odds are they appoint somebody politically well-connected, and that person then gets to use the "chairman of the board at alphabet" title to lobby various governments in Google's favour, rather than they appoint somebody who is going to do any serious oversight of alphabet's executives.