For my (still pure-concept) Warlord proposal, the major subclass might be called your "Combat Focus" or the like, corresponding to the methods the character favors using. This could include things like "Sapper" (military engineer and trickster), Skirmisher (lightly armed and armored high-mobility combatant), Vanguard (heavily armed and armored front-line leader), Mastermind (delegation and facilitation over direct participation), Knight-Enchanter (specialist in effective use of battlefield magic), etc. This would be where the bulk of your subclass mechanics arise, as this is the overall theme or concept you're wanting to implement.
By contrast, the minor subclass might be called your "Leadership Style." This sets which of the three mental ability scores is your Leadership modifier, fueling the rest of your class and subclass abilities, and giving an overall ethos rather than a specific method. The three concepts I have as of now are Bravura (Charisma-based, high-risk/high-reward), Resourceful (Wisdom-based, extra healing and/or status-removing effects), and Cunning (Intelligence-based, extra mitigation and buff effects).
Between the two, you get a class flexible enough to potentially want any of the six stats. A Bravura Vanguard Warlord will want Strength, Charisma, and Constitution, as she's mixing it up on the front lines and intentionally taking risks for payoff. A Cunning Mastermind will want Intelligence, Wisdom, and Dexterity, to spot weak points, foresee enemy behavior, and avoid incoming attacks. A Resourceful Skirmisher will want Wisdom, Dexterity, and Constitution to apply both triage help and targed harrying, able to get in, deal with the immediate threat, and get back out again. Etc.