This is going to sound like a "why not both?"

Add a new geomancer job and make it tank, healer and dps(melee/range). Whichever off-hand gear is equipped determines if it's tank/healer/ranged/melee. Then just re-map the 1-30 skills as if they were half pld, half whm, half mnk, half brd. That way everyone who switches to it at least "gets" the idea of how it works and aren't alienated by high level skills that can't be used in levelsynced 1-30 content.

Unlike the SCH/SMN/ACN, basically the entire skillset is themed to the off-hand gear. So they have to wear tank gear, healer gear, melee gear, or range gear. The game already supports using the same main hand gear for multiple jobs. It is not a stretch to move the vital main traits to the off-hand gear to make it work.

Or they could just increase the capacity of the "job stone" inventory and make more than one job stone, thus 4 separate jobs need to be leveled.

The game really does need at least one job that you can play as tank/healer/dps without having to level three separate jobs that play differently.