i don't have a lot of experience with lego advent calendars, so i'm struggling to think of how lego can make them better other than providing more exclusive minifigs and/or making it cheaper. maybe they can put in some of the harder-to-get minifigs that only come in expensive sets, like oola, or queen amidala. they aren't technically new, but they aren't the same old cannon fodder like battle droid and red shirt troopers. but on the other hand if they did that, there'd be no reason to get the other sets.
brick built droids? weapons rack?
energon cubes? i don't know. micro models seems to be the only viable option. if they made them bigger and beefier, and increase the piece count without increasing the price, i think that would make it seem more worth it. but as it is now, with the the increased cost, it's hard to justify.
unless you have a tradition of doing advent calendars and like the star wars ones, then you're just screwed.