Let's face it, as we see things now, there are some core DnD 5e features that are missing from the game as is so far. Some of these are understandable, as we'd never expect Larian to include all of the extra subclasses and such provides in resources like Xananthar's Guide to Everything, Tasha's Cauldron of Everything, and the like. On the other hand, some of these things could be good to include, and there are some core PHB features and subclasses that are woefully missing as of now. I'm going to list a few that stand out to me, but please do add!
1) Variant Human
Let's be real, PHB standard human is really, really, earth-shatteringly bad. Sure, they get more stat points than anyone, but the fact is that most of them will be bonuses to dump stats, which is not helpful. There is no reason to play a human under these rules when every other race has cool unique abilities, skills and lots of stat points still. Variant human, on the other hand, is great. +2 to any ability, a feat, and a skill are great. They could always exclude Ability Improvement from the list of feats for a human and it'd still be good, and would still give other races an advantage in terms of stat points. The skill is also especially good given the way the game is set up. As the game is currently written, only Astarion has a perception (a critical skill), and no NPC has any charisma skills (though Wyll can get them if you run him). The end result of this is that it is highly advantageous to take some of these skills regardless of whether or not your character focuses on charisma at all. Yet you don't want to take away from skills you'll be really good at, hence the dilemma. Right now I tend to make Wood Elves on almost every character in part because they gain perception and stealth and thus get four other skills that are suited to class. Axing the standard human for at least newly created characters in favor of the variant human would be a massive improvement to the game.
2) Half-Elf Skill Versatility
One of the main reasons to play a half-elf is skill versatility, and this is magnified all the more given the considerations above. But instead of this standard half-elf ability, we are given the much worse half-elf subrace options, of which only the wood elf provides any real value. Restoring this critical ability to half-elves is critical in making them desirable to play. If they wanted to streamline and provide some limits on how this is worked out at character creation, they could always just give the choice of a second background, which would also fit half-elves nicely. A half-elf who is both a sailor and a criminal or a charlatan and a guild artisan would make plenty of sense in the game world.
3) Champion Subclass
Eldritch Knight and Battlemaster are powerful, but complicated subclasses. Champion is easy and still very powerful, plus critical hits are great fun. Ideally there are a wide variety of people who will play this game, many of whom might be new to either DnD generally or to fifth edition. Why not offer the easy button?
4) Oath of Devotion
Paladin hasn't been released yet in early access, but community members have used mods to look under the hood and see how it is progressing. Right now, only Oath of Vengeance and Oath of the Ancients are available. These are nice niche options, but don't feel very classic. Why not just include the classic Oath of Devotion which is right our of the PHB.
There are doubtless many other things, and please post them below! I will add more items as they occur to me.