Here comes quest-master to the rescue.
Here are two suggestions that when combined may be simpler and more appropriate to use than the current systems being tested in Pathfinder Alpha. If you guys have anything better or comments on improving this further, let's see it.
1. For full BAB, use a d12. For 3/4 BAB, use a d8. For 1/2 BAB, use a d4.
2. Give characters maximum class hit points + Constitution score (NOT mod.) at 1st level.
As a result, characters are hardier and much more likely to have appropriate hit points by class.
For those choosing not to roll hit points, 12, 8, and 4 become 9, 6, and 3 for 3/4 hp and 6, 4, and 2 for 1/2 hp. The division is easy and balanced.
Having a 10 or less Constitution score will also be much less fatal at lower levels. Higher levels usually see magical protections and class abilities that can make up for low Con anyway so the focus on what to do with the effect of a Constitution score should be on the lower levels. This helps players make the kinds of characters they want to play without worrying too much about the Constitution score dictating survivability.
P. S.
If you're worried about the Barbarian's advantage of having a higher hit die going away, just give the Barbarian a good Reflex save (if they have uncanny dodge, it only makes sense) and/or give them the following class ability for their toughness:
Iron Endurance (Ex): Whenever the Barbarian is allowed a Fortitude saving throw to halve damage taken from an ability or effect, the damage is instead negated on a successful save.
There is a non-OGL class ability starting with the letter "M" that uses both Fort and Will but this one has a different name and only uses Fort so it should be legal to implement. Just Fort or just Will doesn't seem to appear on any of the classes in the WOTC books. If it does please mention which book you saw it in and which class.
