HD ratings for monsters have indicated two separate things since the earliest system appeared. HD indicate both how well a creature attacks and how much of a beating it can take. I think we can add a great deal more variety to our monsters if we separate those two in some fashion.
The easiest way I see to do this is to simply use hit dice of various sizes. It’s not a unique thought; indeed, Gygax himself indicated using D12s for the largest dragons would be a good thing in an online discussion. I don’t see that the idea has been used much, though, and I wonder why. (It very well may appear in works I don’t have in my library. I’m not current on bestiaries. I don’t see it in online discussions, though.)
A 4 HD creature with hit die size of D4 is much different than one with a D8 hit die. They both fight with equal ferocity, yet one is much easier to defeat than the other. Such discrepancy can add a bit of flavor to encounters, with easily dispatched critters still able to drain hp from PCs if the party isn’t careful.