So, I've heard the devs are working on Temple of Elemental Evil and want to give more incentive to run the optionals.
While I am waiting in a bugged out Von5 for a GM to answer, this popped into my head (which means it might have already been suggested):
If you want people to run the optionals instead of zerging to the finish, why don't you tie in optionals with increased drop rate in the end chest? For example, doing X optionals increases the chances for an item to drop in the end chest by Y%.
Just a suggestion.
Get ready to criticize in 3, 2, 1, GO!