The exhaustion track is inadequate in that it is not integrated with your resources.HP? Exhaustion? How do you find those inadequate?
There is no rule for a caster or fighter pushing themselves further beyond their normal capabilities.
If HD are like inner reserves which you can use to 2nd wind, then surely that should tie to your exhaustion track. If you are exhausted your HD should drop to 0. And if they are providing you a 2nd wind of sorts why can you not expend them to cast spells or perform combat maneuvers...etc
The integration of it all is lacking and that lazy design bleeds then into the rest mechanic.
You'd think after 40+ years they'd have done something about it.