I know this is a dishonest bait thread but I'll give an honest answer.

>playable races
Remove all of them except Human, Dwarf, Elf, and Halfling. Orcs, goblins and the like still exist but they're monstrous and they can never be playable.

>classes
Remove all except for Fighter, Rogue, Cleric, and Wizard. No more subclasses. Subclass style abilities are just feats.

>combat
Far more lethal and scarier. Hit points are miniscule, the most powerful of ancient monsters shouldn't have more than 60-70 hp, and high level characters should never have more than 20-40 hp depending on their class. Going down isn't just a funny little nap instacured by healing word, dropping to 0 creates long-term or permanent injuries that require intensive tending or powerful healing to make right.

>magic
Less spells, more versatility within them. Command, for example, can easily contain Hold Person and Tasha's Hideous Laughter and Sleep effects. More emphasis on having everything more or less available at level 1, but casting with higher slots opens more options, bigger AOE, damage, targets, etc. A level 1 fireball is 1d6 with 5ft radius, level 4 4d6 with 20ft.

There's a lot more finer details but those are the biggest changes that would result in an immediate and much better game.