I know. "Fix" implies there is something "wrong" and that's okay. We are allowed to not like the way a thing is done in D&D (or any other game). But it also suggests something positive: that by tweaking or changing a thing, we can make the game better for our own purposes.
So here are the rules: present a "problem" with D&D (any edition will do) and explain why you feel it is a problem (this part is really important) and the either suggest a fix, or ask for a fix from fellow community members.
For example, one thing in 5E that I really find to be a problem as a GM is how poorly the action economy is balanced for "solo" creatures. A PC party of 4 or 5 characters punches WAY above its weight class against solo monsters, even in Lairs and with legendary actions. A good part of this has to do with the 5E math -- solo monsters don't hit especially hard and so they aren't terrifying in that "stay away from it or you're dead!" feeling that helps keep the PCs at bay. On top of it, PCs can really pump out a lot of damage when they want to and solos, which are usually just big bags of hit points, don't last long. All that said, the fight against one massive foe is a fantasy staple and I want it to work -- and not just for epic boss battles. There's no reason a random encounter with a giant or whatever shouldn't be viable, too.
One thought I have had to fix this is to treat a big creature like a group of creatures that all stay close together. Like, if the dragon were it's head, it's tail and its torso/claw routine. So the head not only gets to act independently on its own initiative, it has its own list of abilities, its own reach and range, and its own hit point pool. The same for the other parts. But while I think it is a neat idea for a dragon, I don't know how it would translate well to a giant or other creature without lots of "interesting parts."