At a minimum, a Mentalism-based Ranger should have Channeling restricted and Heavy Armor costs lowered. He'd be more powerful, as an armored spell-caster, subtle spell-caster, and better Open lists available. He would also be closer to that famous Ranger in the famous book, who often does things that could be interpreted as Mentalist spell-casting, but doesn't go around reciting spells so much. This would be another method to slightly upgrade the Ranger, if you think he needs it.