Although Counterspell has been a true contender in trying to substitute the discard spells, I think Thoughtseize is for Modern what Force of Will is for Legacy and Vintage
Ugin is simply not a good fit for the deck at the moment
The deck felt unbeatable, and though WW strategies are traditionally thought of as "bad" in Cube (see Andy Cooperfauss' Rebel errata as a possible fix), I haven't found this to be the case with my current build: My most recent post in the "3-0" thread illustrates the power of what a good friend calls "Death and Taxes cards." Check out this lineup of 1's: Champion of the Parish, Champion of the Parish, Dryad Militant, Dryad Militant, Loam Lion, Loam Lion, Figure of Destiny, Student of Warfare, Isamaru, Hound of Konda, Mother of Runes, Weathered Wayfarer, Wild Nacatl See
In this case, that meant the Arabian Nights , Antiquities , Legends , and The Dark expansions