Here are some of mine I've been thinking of:
Less linearity: One of my problems with SM/USUM was that it felt more like I was just "riding a roller coaster" rather than just exploring on my own. I'd be forced to follow this linear path set before me until I hit that flag that interrupts the gameplay. Gen 5, especially BW, was a circle. And Gen 6 was a flower. It just felt like there wasn't much reason to revisit many towns/cities after leaving them the first time. The Galar region, as I see it, kinda seems like we'll be going in a straight line upwards. Hopefully I'm wrong, and that there will be a lot of extracurricular things/opportunity to explore on the way to the next goal, a la Gens 1 through 4.
Less hand-holding: It's been 8 generations. I know how to catch Pokemon already. I know it's to cater to the ones just starting out, and that's why it should be like GSC: optional. Be like the Dude. Another suggestion is something like the Teachy TV. That way, help is there for those that need it, and those that don't can go on about their business.
Customization: Hoping this returns, and is just as extensive (or maybe even more) as it was in X/Y. Maybe even allow the ability to change the color of all clothes, not just white ones.
PC is less smiley all the time: Please let our player characters emote.
Return to a past region: What with the whole thing about Galar possibly being near Kalos due to irl location and Melmetal, it'd be nice to be able to revisit it, maybe post-game. It'd be a good opportunity to "complete" Kalos, as X/Y felt kinda... the opposite. I've always loved the idea in games of revisiting past areas you've been in after a time skip, which is one of the reasons I love GSC and BW/B2W2 so much.
Streamlined trade/battle system: Essentially X/Y's PSS. Everything was right there on the screen, no bells and whistles. Quick and painless.
Seasons/Seasonal events: One of the reasons Gen 5 is my favorite is because of seasons being a thing. It just made the world feel more real. Have certain events, items, or locations only available during certain seasons, like Casteliacones during summer or something to that effect.
Adjustable difficulty/harder difficulty in general: While I wasn't too big of a fan of USUM's direction, I did like their attempt at difficulty (certain totems, Ultra Necrozma). B2W2 also did this pretty decently with the Key System, though I'd prefer if all difficulties were available from the start.
That the new gimmick isn't overwhelming: Essentially my problem with Z-Moves, which were for the most part a free insta-kill button. Hopefully whatever this new gimmick is adds a new level of strategy to the gameplay other than "netting me a free KO". I've seen the whole Armor Evolution rumor being tossed around, along with it maybe doing something with buffing either offensive or defensive power (hence, Sword and Shield). I think that'd be pretty cool.
Post-game content: It doesn't necessarily have to be a Battle Frontier of sorts (though I'd really love that), but I'd like something to keep coming back to after completing whatever bit of story happens after I become Champion.
speaking of story...
A story that knocks my socks off: I can't say I play Pokemon purely for the story, but BW/B2W2 managed to tell a story so good to me, I went out and bought the other version just to experience it again. Despite the hand-holding and barrage of cutscenes, SM told a good story too, as well as creating, imo, one of the best antagonists in Pokemon history with Lusamine (before ruining her entire character with USUM). I'm not saying it needs an extensive, uberdark narrative, just a nice good story that's reasonably paced, while also giving me reason to care for each major character.
Something something Arthurian mythology: This is just a personal wish of mine, but I'd like maybe just a hint of something as a fan of Arthurian mythos. Take the legendaries, for example. Maybe the Sword legendary could be based on Excalibur, and the Shield legendary based on Pridwen. And the 3rd legendary could be based on Rhongomyniad which could hint at a Pokemon Lance/Spear or something.
...I have more, but I'll stop before this wall-o-text gets any bigger. I was glad to hear in the direct that they're challenging themselves to try new things, and I'm really hyped to see what else is shown off. I feel like a good 80-90% of what I listed probably won't happen, but speculation is one of my favorite things to do leading up to the release!