Here's something I'e been thinking about for a while: using hit point bonuses for monsters as combat bonuses. There's a hint of this in Monster & Treasure: "Attack/Defense capabilities versus normal men are simply a matter of allowing one roll as a man-type for every hit die, with any bonuses being given to only one of the attacks, i.e. a Troll would attack six times, once with a +3 added to the die roll." This seems to have fallen by the wayside, and those trolls with 6+3 hit dice were later reduced to using the 6-8 HD" column on the monsters attacking combat matrix.
But there's a seeming survivor: the water weird. It attacks as a 6 HD monster, but has lower hit dice. Today's post on the Save or Die blog reminded me, though, that the water weird's hit dice listed in the AD&D Monster Manual are specifically 3+3. Perhaps it's a coincidence that the HD and hp bonus added together equal the effective hit dice, but I'm thinking it shouldn't be, especially when using the older one attack roll per combat turn approach of the LBBs.
My possible new rule: a monster's effective hit dice for attacks equals the total of its actual hit dice plus any hp modifier. This allows us to easily describe monsters that are easy to kill but dangerous for their size: a 2+7 HD creature would have no more than 19 hit points, but would attack as a 9 HD monster, which would normally average 31 hit points. It also allows us to describe monsters that are weak in combat but hard to kill: a 9-7 HD creature would have an average of 24 hit points and as many as 47, but would attack as a 2 HD monster.
I'd also like to keep my house rule of modifying morale rolls with the hp bonus. That 2+7 HD creature is likely to fight to the death, while the 9-7 HD creature may bolt at the first opportunity.