Whenever Finneas attacks, put a +1/+1 counter on each other creature you control that's a token or a Rabbit. Then if creatures you control have total power 10 or greater, draw a card.
Deploy Finneas turn two and flood the board with Rabbit and token creatures, then attack every turn to pump your team and refuel your hand. You snowball a wide board into lethal damage while drawing extra cards once you cross 10 total power, which is trivial in a counter-pumped go-wide deck.
Cheap two-mana commander with vigilance so he attacks and still blocks
Built-in card advantage engine that's easy to trigger in a wide deck
Anthem-style counter growth makes tokens and Rabbits scale fast
Operates in GW, the best colors for token generation and counter synergy
Heavily reliant on the board state; vulnerable to board wipes
Counters are permanent but creatures dying still loses your investment
Limited interaction in GW (no counterspells, little removal)
Can stall if Finneas is repeatedly removed and the board is sparse
Anthem that buffs your token swarm and helps you blow past 10 power for the draw.
Add token doublers like Anointed Procession and Parallel Lives plus proliferate engines to accelerate the counter snowball. Include protection such as Heroic Intervention and Flawless Maneuver to survive board wipes that prey on go-wide decks. Tighten the mana with fast token producers and an Overrun-style payoff so you can close before opponents stabilize.