The following is presented on the argument that 'hit dice' are required by the game to serve two purposes: 1) to designate the defensive power of the monster; 2) to designate the monster's attack dice.
Rather than creating a distinct characteristic for defensive strength, which would need to be assigned for every monster, I propose changing the rule which states that a d8 must be used for the hit dice of every creature. Instead, that the actual die (or dice) rolled for the monster should be comparable with the monster's mass, so that creatures with greater mass would have more hit points per die, while those with less mass would have less hit points per die.
Hit Points per designated Hit Die
Thus, the above table indicates that for different masses a single hit dice would indicate differing amounts of hit points. Thus cattle, which the Monster Manual lists as 1 to 4 hit dice, and weighing 500 plus pounds, would have 1d12 hp per hit dice. An elf, which weighs only 100 lbs., would have 1d6+1 for hit points. A dwarf weighing 150 lbs. would have 1d8, as would a human. And a buffalo, weighing one and a quarter tons and having 5 hit dice, would roll 5d10 and 5d12, total, for hit points.
PROS:
1) It gets rid of the half hit dice and the quarter hit dice exceptions, since a 50 lb creature like a kobald would now have 1 HD but still have 1-4 hit points. As well, a creature like a brownie that attacks like a 9th level could be re-listed as 9 HD, and still only have 9 hp.
2) Massive monsters are now truly massive. I have play tested a 7th-level party fighting a remorhaz with 230 hp, and while it makes for a long, trying battle, it is by no means detrimental to play. In fact, it makes the bigger monsters truly difficult to kill, even for higher level parties, since it allows the monster to withdraw and fight another day, while still having more than 100 hit points, drawing out ordinary combat scenarios into lengthier party-monster duels.
CONS:
1) Many cherished hit point to combat ratios would be rewritten and skewed out of existence. Traditional aspects of the game would be changed dramatically.
2) Most will be unhappy with certain low-mass creatures (such as elves, sprites, dryads or stirges) being downgraded in hit points. Overall, it makes tinier creatures easier to kill, which will not fit well with many harassment scenarios. It may require in some cases that these creatures be given additional hit dice to compensate for their mass, to reflect their fighting ability, which means redesigning the Monster Manual to some extent ... something which was not desired.
3) The unwritten rule about 400 hp being the ultimate high limit would have to go. It's possible that numerous supernatural creatures would have to have more than a thousand hit points to compare favorably now with earthly creatures, and this could drive combat into even lengthier periods would could hurt game play mechanics. In that case, it would be a negative side-effect.