Here’s the deck I used for the Festival: Mountains of Mana event on MTG Arena. The format was Treasure Emblem standard (standard, but at the beginning of each upkeep you get an artifact token you can sacrifice for one mana of any color). With the format providing built-in mana fixing and acceleration, there was no reason to not play a five-color good stuff deck.
And if you are playing a five-color good stuff deck, there’s no reason to not play Jegantha, the Wellspring as your companion. You give up a few things that you might usually put in a five-color Niv deck, such as Kaya’s Wrath, but you don’t really feel it with this aggressive build. In some games, Jegantha is just a five drop that you always have access to. In some games, Jegantha lets you double- (or triple-) spell
Hero of Precinct One does a lot of work here. Half of the deck resembles the classic Esper Hero lineup— Hero, T3feri, Oath of Kaya, Thought Erasure, and Thief of Sanity. Half of the deck resembles Simic Ramp— Growth Spiral and Uro. The result is the best of both worlds.