This is an option I'm considering, which I'd like to hear some opinions on. It's not a definite rule, just something I'm considering.
Basically, the idea would be that not every class level comes with a hit die. You would start with a certain value of hit dice based on your race or size (current idea is small get 1 HD, Medium get 2 HD, Large get 3HD, and so on.) Heavier fighter classes would get hit dice every level, because their training and advancement comes with some inherent toughness. Caster classes, on the other hand, would get them only sporadically, or even barely at all. The HD gained would be based on your base class, setting up some importance for what class you start with.
This has the benefit of making Con more effective for fighter-types. The discussion point I've seen in the 'preferred features of old editions' thread was that by giving every class con benefits to level, it neutralized some of the distinction between a fighter and a cleric, making the cleric almost as tough as the fighter when they both have high con scores, moreso when a caster has a higher con because they're otherwise SAD. This is one approach to reintroducing that concept, after a fashion.
This would also allow me to have the Toughness feat grant an extra HD, with matching con bonus. (Personally, I like this as an idea anyway.)
The reverse optional setup would be to have the option of having certain classes gain multiple hit dice per level, with matching extra con bonus. Or perhaps a combination of the two, where Mages and Adepts might get HD only every 2 or 3 levels, while Brutes and Knights would get two HD per level.
So what do you guys think?