- Monsters: From the first list of ideas Michael gave, I found that there were very few creatures. I'm not sure if it's because of the oversimplification of some groups (Dinosaurs, for example) or because he's trying to stay clear from the monsters that appear in D&D and the such. It's a great idea to get out of the comfort zone, and even greater to be prepared to make mistakes. I applaud this effort. However, I'd like to suggest that you look into other Stone cultures (Latinamerican, Australian, Celtic). It might expand both your creature list as well as what you could add to the game for mechanics and/or other stuff (plots, loot...)
- Magic: Not so sure how a Portal school healing spell could work, the ideas in general are basic and might need a bit more refining in the literary sense. As an example: Would Earth magic allow you to create sentient warriors (golems)? You only mention it can alter earth and rock, but I'm guessing you could expand on those, because it is really a big difference if the only thing you can do is something close to Earthbending (launching stone projectiles) but you can't do Earth summoning, so to speak. The rest of the magical schools sound great, and I can think of a plethora of spells for each one (healing rain, enemy shift, fire link, tsunami endurance, and so on).
- Classes: Once again, the first list could have done with a little refining, but with your revision (removing the Alchemist class, and adding the Marauder and Necromancer) it makes it look more in-setting. I like the various ideas you have for most of them (except the Bard and Assassin).
Why don't I like the bard? Well, in Stone ages, and in most stone tribes, the one who "sang" stories and legends was the Shaman or the Tribe Elder (Usually the chief, as well). As for the assassin, I feel like it's too advanced a term for a world of Stone. Sure, the Trickster is an advanced term as well, but it sounds more feasible to have a guy who can make tricks and traps to escape and confuse the foe in a Stone Age setting than a guy who's a trained killer. At the same time, in a Stone Age setting, almost everybody was a trained killer of sorts (All you had to know is how to hit your foe over the head with a big rock hammer) of both men and monsters. - Tribes: The historical approach might prove quite difficult, and you're sure to piss someone off if it's published and you made one tiny mistake. However, it might also turn out to be easier in the creative sense, since you're already a step ahead in the basic design of the tribes, and you just have to add what makes if fit the game and setting. Creating your own tribe might be great as well, considering it allows for you to fully cutomize your character's traits. However, as I had stated before, it would be great to have a general idea of "possible" tribes (like, say, some sort of Northern tribe would usually have a natural resistance to cold, or a natural bonus to strength).
So those are my thoughts on Stone Lords.