We're talking after Hanover was raised to an electorate but before the Napoleonic Wars, at which point the dignity started getting thrown around. Here's the ones I think are most likely:
Austria: Most likely candidate if the Habsburgs try to get another vote for themselves beside Bohemia's.
Salzburg: The only archbishopric in the HRE that was not also an electorate (maybe, I'm not sure about Besancon). The archbishop was Primas Germaniae.
Wurttemberg: The electoral college was skewed toward the Catholic states and Wurttemberg is among the more important Protestant ones, so it could be made an electorate for the sake of balance.
Hesse-Kassel: Same as above.
Brunswick-Wolfenbuttel: Tried to get the electorate at the same time as Hanover.
Any other proposals? When were the best opportunities for a new electorate?