Thats the issue though.
If a diver with their usual 50/50 build can dive in and kill the marksman/mage through peel then marksmen/tanks/mages and supports will flip their tables and go nuts.
If the diver cant kill anyone and blows up too quickly then they go nuts.
If they become too tanky while keeping "decent" damage then the tanks go nuts, as do the assassins.
For a diver to not be considered impossible to balance they need a niche like rework Irelia being a anti ranged diver, she has great diving and gapclosing but she is kinda bad in a straight melee fight compared to the more oldschool ones.
Only reason that literally everyone doesnt hate her is exactly that, right now only mages and marksmen do which is probably fine.