North Star and Fist of Suns
I do not believe it works due to the wording of the rules regarding costs.

202.1. A card’s mana cost is indicated by mana symbols near the top of the card. (See rule 107.4.)
601.2e The player determines the total cost of the spell. Usually this is just the mana cost. Some
spells have additional or alternative costs.

Fist of Suns has a static ability that creates an alternative cost for all spells that are played. This alternative cost if {W}{U}{B}{R}{G}. However that alternative cost is not the mana cost which is still the original printed symbols.
For this reason when both are out if you attempt to pay the alternative cost of {W}{U}{B}{R}{G} you are no longer paying the mana cost, so the ability to pay mana as if it were mana of any color no longer applies.
Thus after activating North Star with Fist of Suns in play you can either pay {W}{U}{B}{R}{G} or the CMC of the spell, but cannot choose to pay {5}.