They should just make Protect a WHM only ability that works similar to the PVP version. Increase defense or reduce damage taken for a few seconds for all party members on a 60 seconds cooldown or so. Like an AoE Divine Benison.

And as WHM already has less buttons than AST and SCH (as WHM I have space for all cross role skills while as SCH and AST I need macros to switch the hotbar slot between some of them), they could actually also make Protect AND Shell as separate abilities, one for physical and one for magical damage. I don't think that it would be so overpowered if the buff wasn't that strong. AST and SCH both have access to regens AND shields, so why should WHM not also have an AoE shield-like ability?