Hiya!
What I'd like to see is more "one-off" adventure modules. Goodman Games had the DCC line and that was amazing! It seems WotC is focused on "save the world...again!" hardback adventure paths. Thats great for people who like to do that, but one of the key selling points for 5e is it's ease of use and "loosey-goosey" play style (the whole 'rulings, not rules' thing). The DCC adventures fit perfectly with that style of play. A module that can be given a serious skimming (say, half-hour) and then the DM can sit down and run it. No multi-hour or multi-day memorization of convoluted plots, or detailed NPC backgrounds, etc. Just "Here's a couple of maps. Here's some random encounter charts. Heres a couple of ideas to get the players interested in it. And here's the stocked dungeon/ruin/castle/forest/swamp, etc.".
One adventure that my group and I REALLY enjoyed was the "Tomb of the Iron God". Admittedly, I used it with a little-known game system called "Dominion Rules" (free, online, kinda old), but that one module carried us for about a dozen sessions (about 3 months). I bought the PDF version, which was about $5 I think. And *enormous* cost-to-fun ratio, I must say!

If you guys could pull off a bunch of modules like that...then you should offer a monthly subscription option like Paizo does for their PF stuff. I'd sign up in a heartbeat!
Ok. After the "old style modules", I wouldn't mind seeing books dealing with stuff other than more monsters, spells, magic items, or (especially) stuff that is already optional in the core 5e books (yes, Feats, I'm looking at you!). A book dealing with sea-born adventuring. Maybe one dealing with Keeps/Towers/Castles. Another with underground ecosystems and what they may look like in a fantasy setting. Maybe a book with how to build a house, manor, inn, tavern, or something "common" and how to keep it running. Perhaps something to do with arctic wilderness survival and exploration. Pre-Historic stuff. Desert adventuring. City/Town/Settlement construction and information about urban adventures. Basically, all the stuff that players end up in or doing, but a 5e DM has very little info to help him run it in a believable and consistent manner. Just slapping together yet another book of monsters/spells/feats/PC-add-ons does
nothing (or virtually nothing) for the beleaguered DM. I want books that spark my imagination and get me to want to write some adventure (or buy one from you guys!) set in the Swamplands of Peool, or deal with the political machinations of the nobility in the capital City of Games. That's what I want.
EDIT: Just saw your post about "price, format, etc". Softcover for 'supplemental books'. Old 1e removable-cover style printings (with mono-color maps!...we don't need full-color fancy-pants maps that up the price by a huge factor), for adventures. Everything should also be available in PDF (at a much cheaper price), as well as ePub and Mobi, and/or available as a combos.
EDIT-EDIT: Oh, also the font should be at least 10-point, black on white. And... NON-GLOSSY PAGES FOR EVERYTHING! Sorry for yelling, but the glossy-pages thing that's been going on for the last decade drives me up the wall. Lighting has to be *perfect* to read it easily. I have old TSR books and adventures that I bought new, like my World of Greyhawk Folio, that is *still* together and usable...and I have hardback, full-color, glossy paged books I bought last year that are warped, falling apart, and have smears/smudges/wrinkles all through them. Just saying...
^_^
Paul L. Ming