Hit Points will remain the same as specified per class in the LL book.
The big change will be the consideration of mass into Hit Points. Essentially, by adding mass as a factor into determining hit points, we can make giant monsters truly dangerous without having to touch Hit Dice and all the mechanics based off of that. The design change here is made to the dice value of Hit Dice.
For example, an Elephant is listed as 9HD in the LL book, meaning 40 hit points on average. I find it disturbing that you can kill a 6 ton beast so easily. I know hit points are an abstraction, but I want an army that fields elephants to be fearsome, rather than having a mobile target that takes 10 arrows or bolts then dies.
So we start with the assumption that something with roughly the same mass as a human uses d8 hit dice and work from there. As a tentative working system, we can increase the die used every time mass doubles. So, d8 is for an 80kg human, d10 for a 160kg monster, d12 for 320kg, etc. This will create odd dice after this point, but that just means we use a digital random number generator to create D14 or D34. The elephant example (full sized African males weigh in around 5,500kgs) above would have 9HD at D20 for an average hit points of 95. Now we are talking about something fierce! Of course, this monster's to-hit, damage, saving throws versus effects, and susceptibility to effects by HD are all unchanged.
This affects monsters mostly and will require some hand waving for monsters that don't have real life analogues in which to base their mass. In addition, it does have one major impact on PCs.
All characters Hit Points are determined by summing HPs gained from chosen class and race. Bonuses from exceptional Constitution are only added once. Hit points from race never change during play.
Upon reaching a new level, only class hit points are added to the total.
Hit Points Due to Mass by Race
|Elves, Dwarves, Half-Elves||1D6|
There is a little fudging here since the weights are pretty close to each other, but it works well enough and helps differentiate the player races.
By giving players 2 dice worth of hit points at first level, there is no need to have a house rule for max HPs at first level or for re-rolling results of 1 or 2. With the use of 2 dice and the mini-bell curve, the majority of players will have the same amount of hit points as if they were given one full hit dice worth of HPs with the potential to have much more. A Half-Orc or Human Fighter could, with a little luck, have more than 10 hit points at first level.
This system for hit points satisfies a number of design goals. It integrates the concept of mass into hit points, allowing for a more 'realistic' treatment of large monsters. In addition, it gives players more survivability at first level without resorting to simply giving away maximum hit points for everyone. It also helps differentiate races and offer more meaningful choices.