In my opinion it is a lot easier to balance around an idea that core elementalists have something unique that the other two classes don't. It's hard to buff up traitlines without inadvertently buffing the class specs unless somehow you can introduce synergies that somehow buff core specs the most. If you wanted to go that route, I think it would involve creating synergies between say Fire and Water, Air and Earth, or trait lines you typically wouldn't see together in a Tempest/Weaver build. But as you tinker around with things and balance over time this could become problematic. You can tweak some trait lines for sure though. I think for example certain traits are enhanced because you picked core or are lesser because you picked Tempest/Weaver. Or another thing is attuning to a certain element while a core player gives you an additional effect. Hell, maybe even add an "ultimate" F5 skill, something based on the traitlines you chose. For example if you chose Fire, Water, Arcane, form a ball of energy that explodes, inflicting damage, burning and healing after 2 seconds or some chit. Or if you chose Air, Fire, Arcane, unleash a powerful fire storm that knocks down and burns foes. Something cool like that.