Mark Rosewater's Tumblr question-and-answer blog, Blogatog, has grown in scope in the years it's been in existance. It's a direct line to the most influential R&D member, and some of the discussions there have actually influenced Magic design. So Mark, having answered so many questions, deserves a shot at asking his audience and turning the tables.
When coglineerro asked him recently, "We ask you tons of questions everyday. What's one question you have been dying to ask us loyal question marks?" he responded pretty magnanimously:
If you could snap your fingers and make any product get created and released, what would you want?
The response was, to put it mildly, overwhelming. One user catalogued the more than 250 notes and found this - "The results from most popular to least: Paper Vintage Masters/Legacy Reprints, Un-set 3, Return to Kamigawa, Modern Masters 2 (with larger print run), Planechase, Archenemy, Better MTGO Client/Mac Client, better Commander products, and Conspiracy 2. Others include: gay planeswalkers, reprinting old sets, foil token box, a video game (like Biomass), various tribal products, Bizaralara, various Duel Decks and Return to Innistrad."
These responses may not be a representative poll of all Magic players, but they do show the views of at least players who are well-invested in the game. There are indeed products - supplemental ones - explicitly made for hardcore players, so this fits well enough. Other interesting answers have been calling for Eldrazi vs. Slivers supplemental products, a turn for Ashiok as a spark-devouring main villain, and a Lorwyn-themed supplemental product. Given that most of the suggestions aren't about full expansions, they stand better chances of being printed because supplemental products can afford to have a more niche audience and fewer resources are committed to their design and development.