steeldragons
Steeliest of the dragons
Reminds me of the rule of seven and especially this blog post.
I've also sometimes thought about reducing abilities from 4 to 6. I'd make charisma/persona also be force of will and have Will saves depend on that. Maybe you could come up with something else for Mental(ity). You could also consider having that affect Cleric spellcasting - then each class would have their "own" ability.
An alternative strategy for coming up with four "saves" is unify them with AC. You'd have four defenses: e.g. Physical (normal AC), Dodge (touch AC/reflex save*), Endurance (fortitude save/constitution) and Mental (will save).
*This distinction never did make much sense to me.
I do like that idea. Wasn't really thrilled with meshing Int. and Wis. together. Incorporating "sense of Will" into Persona (which does work) and making that the Cleric-based magic ability works great for me.
Didn't love the term "Mental" either, so let's make that "Smarts." This works as the ability for Magic-users magic ability, "book smarts" and the acquisition of skills (important for everyone, but especially Thieves), as "street smarts."
And thank you for reading my mind and breaking down the 4 Defenses/Saves for me! "Endurance", of course, will have to be linked to Persona (force of will to keep going?), since we don't have a "constitution score", whereas "Mental" would, somewhat clumsily, be linked to "Smarts".
So, Abilities: Strength, Agility, Smarts, Persona
and Defenses (as opposed to "saves"):
Physical (AC): your defense against physical attack. I'm gonna propose your armor and Agility modifier (if any)
Reflex (agility +smarts based): your defense against falling, for dodging attacks and avoiding (or, at least, lessening) area-effects
Mental (smarts + persona based): your defense against mind-effecting magics, "seeing through" illusions, fear and confusion effects
Endurance (strength + persona based): your defense against poisons, physical-effecting magics (polymorph, slow/haste, "death magic" etc.), exhaustion and environmental hazards
As has always been the way, from Basic on through, these are by no means exhaustive in their coverage and DM "best/closest option" adjudications are expected to be made in play. An area effect "mass charm" spell, for example...is it Reflex or is it Mental? DM decides or, if being kind, player uses their best option.