I think Stoneskin should stay single target only.
However, what I think they could do is give us something like Scholar's stratagem Accession in FFXI (or make it for ACN/CNJ but also available for cross class). The ability makes the next spell AoE. Sounds a bit overpowered, but it would be useful in hastening the start of a dungeon/fight, and can potentially save your party from a wipe if used at the right time. It would, of course, be one of those last resort spells with a long ass recast time and probably at half effectiveness so it's not too cheap. Just a thought.
Or they should give WHM Afflatus Solace from FFXI. Cure gives a slight Stoneskin affect for 25% of the cure amount = cureskin! Would have to be less (like 10% or less) since we easily can cure 1 for like 1k+ hp though. 250 buffer for each cure? Now THAT would be overpowered.