And as a different but intrinsically related question, should an OSR style fantasy game strive to focus on a specific fantasy genres or a wide variety of them? I'll get to how that's related in a moment.
The base idea of this system is to give more depth to character choices by making it so their race has more significant effects on their character track. Each race, in addition to having normal modifiers to statistics, would have a racial Hit Dice. Humans would have a d8, sturdier races like Dwarves and Orcs would have a d10, and Elves, being all wiry and frail, would have a d4. This was inspired mainly by ADOM (and a bit by Dungeon Crawl Stone Soup, which had similar mechanics, though I've never played) where races would have their own advancements and modifiers to things like spell learning, hit point advancement, natural Hit Points/Power Points and hit point/power point regeneration. Some races had their own special abilities too; Mist Elves had a modifier that cut their total Hit Points in half, but also an ability that halved all physical damage they took.
The idea here is that your Elf could have a bonus to their Constitution, but this was offset by a low natural Hit Point growth for their race. Of course, they'd get a class Hit Die, which could help offset this inherent weakness if they chose something like a Fighter. However, they could also choose to be a Wizard, which would come with a low Hit Die as well. This gives a huge variety; it also allows for a bigger difference in character statistics. For example, you could give the Elf more bonuses and have it offset by their Hit Dice. So your Elves could have a bonus to Constitution, Dexterity and Intelligence (and a minus to Strength, which I think is fitting). You could also have different bonuses for some races (maybe Orcs automatically grow bigger and stronger every few levels, for example).
Anyway, since I've got the primary idea behind the system out of the way, I'm onto the first sticking point; Attributes. I've found this is a little challenging to come to a final agreement too for a fantasy game, since Fantasy style Tabletop RPGs could cover a huge swathe of different genres, as well as the general fit some stats would have for a system. D&D has the classic six, while the game ADOM has nine: Strength, Toughness, Dexterity, Perception, Learning, Willpower, Charisma, Appearance and Mana. I probably won't add Appearance as a stat as it's a tad arbitrary and also very niche, but most of the other stats are fair game.
I guess I could divide this into "Stats I will add" and "Stats I'm unsure of".
What will be added:
1. Strength: Naturally. How many fantastical creatures and heroes are known for their great strength? If I had to list them I'd be here all day.
2. Dexterity: Another mainstay.
3. Endurance/Toughness: Instead of linking hit points directly to strength, I'll have this be the main influencing factor in Hit Points and overall health, stamina and resistance to physical hazards.
4. Intelligence: Something for skill growth and learning spells. This is where the uncertainty starts. Maybe spell power? I don't know.
All the different sub attributes I'd need to fit into the system would include: Spell power, magical energy reserves, sensory ability, resistance to mental/magical attacks and social interaction rolls. The problem is here is trying not to make a certain stat too powerful or too weak. I plan to let players train their stats over time, so you won't have static stats for your whole adventuring life. The "training" aspect is meant to be like in ADOM or Nethack, where it will occur over time, semi independent of level.
Some ideas:
-Perception: If its functions aren't mixed into Intellect or some Wisdom equivalent, I'll probably have it as a stat. Granted, there's not many different functions for it, which is a problem.
-Willpower: Mental defense as well as how powerful your spells are, if that function isn't molded into intelligence. This one is a big problem, but really I just need to decide how to divide up this potential.
-Charisma: A similar boat to Perception. An overly important function but not really that much versatility. I've seen it folded into Willpower.
-Mana: Not sure if I'll add this or not. It fits in some genres and not so well in others (like Dark Sun, where the stat would be quite strange to have). If I do add it, it would determine magical energy reserves as well as resistance to magic to some degree (and maybe an influence on magical strength, but I want that to be Intelligence or Willpower primarily). I find that it helps for differentiating races based on magical power, but that can just as easily be done with say, a modifier to their Power Point growth. Though a temporary Mana penalty seems to work for creating magic items.
-Wisdom: If I don't add one of the other stats, it might be folded into Wisdom. Though I want to avoid doing that, personally; I'd need to get rid of most stats to justify having Wisdom.
Anyway, I made this post because I needed some outside input. If it seems a little disjointed, that's because it took me many distracted hours to properly type this out, and I only published it in the state it's in now because if I spent any longer I might never get it done. I'm a mess.
If you have any questions about what else I had planned for this system (such as derived statistics, classes, etc) if you feel you need more explanation. Or are interested.
Hopefully your input drives me in the right direction.