Grolnok, the Omnivore is certainly a fun Commander for Historic Brawl. This deck focuses heavily on the self-milling part, plus some Frog tribal synergies.
Of course, there aren’t many Frogs In MTG, and there are even less of them on Arena. However, with some help with Arcane Adaptationand Maskwood Nexus, everything can be a Frog, and trigger Grolnok’s mill ability. Furthermore, we’re also using Shapeshifters to get our Frog count up.
Since this is a green deck, you’ll get a lot of ramp to play Grolnok, the Omnivore, a turn earlier. Its ability also allows you to play lands with croak counters. So, you’ll find a lot of cards like Azusa, Lost But Seeking, and Oracle of Mul Daya, which will help you get multiples of them in play.
All in all, this Grolnok Historic Brawl decklist is certainly a fun one to play on Arena.