D&D 5E Thoughts on a Darksun Cleric

[Coroc]

Steel Greatsword 2d8, d20 greataxes?
I saw upsized weapons as far back as 2E combat and tactics.

Well yes, as long as you disallow feats it will reflect the quasi magic effect of the weapon (GWM is not available to marginalize this damage output) . There are several magic weapons in 5e which give an additional die instead of a plus, so this definitely does not imbalance things.