As a minivan owner, the biggest asset to me is the sliding rear doors. Creates a wider opening to get kids in and out, especially if the are power sliding doors which make loading and unloading kids even easer. Also, you don't have to worry about the rear occupants opening the door and hitting it into a car or garage wall.
My advice...
Change the rear doors to power sliding and have got yourself a winner. The exterior could look essentially the same with the exception of a relocated door handle anda slider track for the door (which could be incoporated with the rear window - See Dodge/Chrysler). This would, I believe, make the vehicle even more marketable.
Again, just my advice