30 card decks are already too small when you can use 6 copies of one card, so decreasing the size would be a very bad thing. They would make combo decks way too easy to pull off. Imagine something like a 15-20 card instosis deck... :p
Increasing the deck size limit probably wouldn't enable any new killer decks, but they would increase the number of fun decks. One of the things I used to do in Magic: the Gathering, when playing with either an AI that was dumb or with human opponents who were either unskilled or didn't own good decks was to use way oversized fun decks without any real thought to whether it would win or not; I'd just throw in lots of cool stuff and silly synergies and things like that. Increasing the deck size cap would enable more of that sort of thing, but I don't think it would enable any really competitive decks, at least in the current meta.