Well, the foil of Dark, otherwise known as "Evil" in Japanese, would probably be "Good". It makes no sense to me somehow. Dark is already the foil to Psychic, so I doubt we need more types. I wouldn't mind a rebalance of the current type chart, though.
Apart from general issues like plot and gameplay, I would really want them to improve the browser functions. First of all, we need decent lists. I don't see a problem in displaying more than six Dream World visitors at once, and for anything else, there is something called "scroll bar". And it would be nice to see more than seven Pokemon in GTS. In fact, when they already bother to show us which Pokemon are currently traded in Global Link, they may just implement a trainding function via browser. With sufficiently small letters for the entries to fit into the screen and blazing fast scolling with our mouse wheels, we can weed those Lv. 9 Zekrom/Reshiram request.