Mutants and Masterminds (and I think GURPS) sidesteps it entirely by having point buy with all the abilities and stats. You don't even have classes.

Or Savage Worlds where you literally build your "class" from the ground up

Rogue: Waitasec, how many boons do you have?!

Warlock: I dunno, a bunch. I lost count.

I knew a wizard that had traded his soul for favors so many times he was effectively immortal. He never went adventuring any more, just oversaw research in our flying screened tower. Since old age was the only feasible way he was going to die, which would cause a war between all the outer planes over ownership of his soul, no one would cause his death. He was 218 when I met him, and he was over 5000 years old, and a demigod of secrets, when I met him again, because of a mixup we had while inventing portal magic. We ended up 5000 years in the past and I went back to the present, but he stayed behind. Pagiathrakatos was an interesting dude. Got a compliment from a dwarf on his impressive beard.

Ahh, an Elder Scrolls protagonist

Big John Constantine energy.

Fiend: Look, I'll take what I can get. Can I get the legs? I'll take the legs. She can have the top part.

Archfey: Did you just call the head the "top part"? That is so fucked up.

Your fighter is gonna be very disappointed when they find out which level they get action surge at

Unfortunately the DMG says that if a character somehow acquires the same feature more than once, only one counts.

No I want to see a programming language with multiclassing. Not just inheritance or Interfaces, but properly being able to make an object from any two classes.

I thought it was about programming and was wondering why the words only half seemed to mean something.

Also any class that gets their subclass at level 1 or 2.

To be fair those are also troublesome for regular multiclassing, or at least they are if you're not using the 2024 "definitely not 5.5E" classes. The paladin with one level in warlock or sorcerer is a perennial favourite for a reason.

This is the anwer. You could always homebrew your own game and try to balance it, and you'd start to find where the game breaks. Play 10,000 games like that, and patterns will emerge. Game developers spend a lot of time playtesting, and they still miss things. Just thinking of a new twist and asking why it doesn't work is like asking why cars don't have six wheels.