I voted Magic Items, New Rules, Classes. Classes is the pinnacle of this for me though, because new classes sometimes come with the other two options.
I'm burnt out on subclasses. They are fun ideas but they can't do much and are usually shackled to the PHB in terms of how strong (read: interesting) they can be without receiving backlash. I also find that subclasses just don't provide the flavor I'm usually looking for in a new class. Even the most radical subclasses, like Bladesinger, while fun, end up still feeling like a wizard with swords.
New classes, however, expand the foundational fantasy the game is built off of. People often talk about how in D&D, you can be just about any Fantasy thing, and new classes is the ultimate way of achieving that. And, with Mike Mearls Patreon, I'm starting to realize that classes don't have to even have subclasses -- they can have totally novel designs that further hone in on the Fantasy of the character in question.
Magic items is probably my second most wanted thing. While I understand that 3-4E had an issue with expected magical items, I find that 5E's absence of really interesting magic items diminishes the feel of Fantasy to me. I cannot think of a single Fantasy setting that doesn't have cool items that characters can use to make their "signature." And if you look at myths, well, its all magic items, all the time. Interesting magic items, magic items that provide spells, magic items for classes, and artifacts most of all are the things I look for most in a new product. But this is because, usually, new classes end up being too conservatively designed for my taste; sometimes, it feels that if a class isn't around the power and complexity of the Champion Fighter, people reject it on principle alone.
Lastly, new rules additions. I'm someone who believes that D&D, while a storytelling game, is still a game and I like to play fun games. I think that this is something that D&D specifically has been lacking on -- new, fun rules for the game. When I look at contemporary war games, which live and die on their mechanics, and most of which are very streamlined, I feel a bit of yearning. On top of that, I really feel like settings in particular should have a suite of mechanics that makes the experience different to vanilla D&D, and really should be close to the center of that setting. I don't see the point in doing a unique setting like Spelljammer without focusing on the Spelljammer aspect all the way, or Dragonlance without a really fun war game alongside it. This is something that, interestingly enough, Tomb of Annihilation did well, by focusing really heavy on the hexcrawl aspects of the game and then the dungeon crawling afterward.
This loops back to my beliefs on class. I like it when settings have either radical changes to classes or have unique classes themselves that focus on the unique aspects of that setting. Alternate class features works well here, in lieu of new classes; if there were alternate features for the base 12 classes in Spelljammer that focused more on astral traveling, spelljamming, etc etc, it'd really make that a unique experience.
Alas, I know I ask for too much.
