Here is my take on how you could make a 2hand enhancement shaman work alongside the dual wield.
Keeping the runes as they are for phase 1 is fine imo since we don’t have windfury anyway.
My plan is to just run Lava burst, Overload and Shamanistic rage while specced into enhance with a 2hander and cast Lava burst on cooldown.
For phase 2 or when they add additional rune slots, they could add maelstrom weapon as one of the new runes. It would allow a 2hand build with maelstrom, overload, lava burst and shamanistic rage to create more of a “spellhance” build.
Maelstrom could be open for both dual wield and 2hand or only for 2hand. Main thing that would set them apart is the access to overload which dual wield can’t get with the current rune placement.
But personally I think there should be a drawback to maelstrom not to make it too overpowered with instant cast spells. Such as a 5-10% reduced damage per maelstrom stack consumed when using a 2hand and 10-15% so a little more reduction when dual wielding to compensate for dual wield generating more maelstrom. That way you can still hardcast a big lava burst or a chain lightning if you want to but in pvp would run the risk of being interrupted.
To me it seems the hardest part about buffing 2hand enhance is due to the nature of windfury adding more melee attacks would run the risk of making them too powerful in pvp. If you add maelstrom to make up for the sustained damage it won’t interfere with windfury and would give 2hand better damage in pve content without breaking them in pvp.
Later if they are going to add a 5th rune slot you could also utilize the Mental Quickness talent from TBC as a rune when 2handing to make the spells scale better.