I don't think the question is about what would make a two-handed weapon powerful enough to be worth using, but rather how much mana is it worth to give up both hand slots. You are basically paying a premium cost with equipment slots, and that should be reflected in a discounted mana cost. If the discount is equitable, then it shouldn't matter how powerful the weapon is.
I personally believe the extra slot is worth about 3 mana. That means if [mwcard=MWSTX2FFQ06]Morning Star[/mwcard] was made two-handed, it would only cost 2 mana to cast. Seems a good trade to me. [mwcard=MW1Q14]Lash of Hellfire[/mwcard] would only cost 5, which again is a trade that I'd make.
If this was the case though, that would mean [mwcard=FWQ11]War Sledge[/mwcard] and [mwcard=FWQ04]Galvitar, Force Blade[/mwcard] were basically equivalent, and they are not. Galvitar's quick attack is clearly better, and I would argue that it's Full attack is better as well. Although, Daze does tend to be valued higher than Ethereal, the versatility of the choice between Sweeping and Doublestrike makes Galvitar superior to me. Not to mention the Cantrip Trait. I think it should be worth approximately 2 mana more than War Sledge before calculating the discount for the difference in item slots.
So, what if Arcane Wonders values the extra item slot at 2 Mana? It might not seem like much of a difference, but it actually would significantly reduce my enthusiasm to play two-handed weapons when we again look at converting the current weapons into that form. My guess is that the actual value of the item slot is somewhere between 2 and 3 mana.