Going to combine two different concepts in this one. One is a combination of mechanics and flavor; the other is pure flavor/story.
Starting with magic items, I want to see the math of the system less dependent on them. That is, I want it to be possible to play a low- or even no-item campaign with minimal, if any, tweaks. The addition of magic items should raise a character/party above the baseline, rather than the absence of them lowering the character/party below the baseline.
Now, to an extent, they’ve already said that’s part of the design philosophy. But I’d like to see it taken a step further. And I acknowledge in advance that this is practically apostasy where D&D is concerned, but here it is…
I want to see +X items stripped from the game.
No +1 longsword. No +3 chainmail. No gloves of Dexterity +4.
A magic item should do something. A sword that bursts into flame on command? Chainmail that lets you turn into an elemental once per day? Gloves that let you throw webs like the spell? Yes! Great! Fine! More of those.
And if some of those also impact the math–such as the flaming sword adding to damage, and perhaps giving a bonus to hit against fire-vulnerable creatures specifically–that’s cool. In fact, that’s more than cool; it’s thematically appropriate.
But items that do nothing but give bonuses? No. Items that give bonuses in all situations? (Such as a +4 sword vs. a sword of gnome-hating which gives a +4 bonus only against creatures half your size or smaller.) No; give me the latter, not the former.
Actually, don’t even give me the latter. Bonuses to hit, even conditionally, should–except in the absolute rarest or most specific of cases–max at +1 or +2. (Damage can be more flexible without breaking the math, but not the actual attack.) If that doesn’t sound like enough to make cool items, well… Make them cooler based on what else they do.
So, enough of that. Alien creatures.
What do I mean by that? Primarily, I mean creatures that are not only grossly inhuman, but whose very concepts revolve partly around their inhumanity. Mind flayers. Aboleths. That sort of thing.
Modules and novels–even those written by good writers, who I really like and really respect–have a bad habit of treating them just like other NPCs. You have aboleth who want to rule. You have mind flayers wandering around the markets of Skullport, or plotting to take over portions of a drow city.
No. No, no, no. I realize it’s hard to write creatures with inhuman motivations–I’ve tried it myself–but if you can’t, don’t use them! There are plenty of other monsters to choose from. One should never fully understand the motivations or thought processes of a mind flayer. The entry of one into Skullport should be momentous, with even the other denizens of the Underdark recoiling in fear and horror–not necessarily because the thing’s more powerful than them, but because they can’t damn well understand it. The notion of a mind flayer merchant is an oxymoron. The notion of a mind flayer "adviser" to a drow noble should fill everyone–including the drow–with dread, because you never, ever know to what ends the thing is actually driving you.
These things are Lovecraft-inspired, so let them be Lovecraftian. I’d rather never see a mind flayer in D&D fiction or adventures again than to see one used in such a way that a psionically-empowered human would have done the job just as well.