Well, thinking back to WarCraft 3, one of the missions that I go back to the most is the Defense of Hearthglen. Mostly because of the bonus quest of the Undead caravan. I always make it a goal to wipe that thing out ASAP, and gather all the best (according to my personal preference) items for Arthas. I wipe it off the map before it reaches the first town, without losing my base in the process.
I know there are people who can cheese it by eliminating the enemy base, or doing much better micro than I can. I’m not the best at the game, I admit it. But that is the sort of thing that this mode needs. Optional objectives that take you off the beaten path, and considerably increase the difficulty of the mode, but which offer a worthwhile reward.
Arathi tried this with the random rares, but they weren’t worth it at all. The rewards did almost nothing, even the elementals died very quickly. These objectives need to reward things people actually want, that makes doing them worthwhile. Raising the iLevel of the final drop, mounts, something. Just make them worth doing and make them so that if you go for them, it makes it a lot harder to win. And change them up so that they’re less repetitive.