Should one be printed in white and/or black, Triad of Fates will become capable of filling this role as well, but as white only very rarely receives access to countermagic, it will be a long time before this becomes a reality, if ever
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
Im a fan of protection spells that leave your board stronger than they were before the effect you needed protection from, and thats exactly the Mutational Advantage youre getting
Many of the best white cards improve cat decks, but you should also include cards like Ajani, Nacatl Pariah, Regal Caracal, Basri, Tomorrow's Champion, and Pride Sovereign
Including cards that cash in on high life totals like Aetherflux Reservoir can be a great payoff for this commander