I am working on a deck that features Progenitus as my Commander and started looking at Fist of Suns. Besides just being good because of the large number of high cost spells in the deck, I was curious how this worked with my Commander on every cast beyond the first. And honestly, I'm not entirely sure how to word this question.
For each cast of the Commander, the mana cost is increased by 2 colorless. The Fist of Suns Oracle text says
You can't pay more than one alternative cost for a spell.
Does the additional colorless cost added after the first casting of a Commander count as alternative costs? Can I use Fist of Suns to make the cost of Progenitus WUBRG every time, or just the first time it is cast? I know that it can't be cast for WUBRG plus the additional colorless, since it seems unreasonable to combine mana costs like that.