Phyrexian Swarmlord
Two of the easiest upgrade paths would be to head toward a big artifact / reanimator strategy with cards like Goblin Welder, Daretti, Scrap Savant, and other huge artifact finishers like Wurmcoil Engine, or play a token- / Thopter-themed deck with the help of cards like Sai, Master Thopterist, Pia and Kiran Nalaar, and friends
it sucks that you lose the planeswalker if you have to deal with a must-kill threat
And while there certainly are expensive upgrades you can make to either of these decks, they both feel strong, think-y, aggressive in their own ways, and will provide many, many games of Commander where you should feel fairly even-toed with everyone else at the table
This made the Izzet Prowess incredibly strong and consistent, with over 40% of the decks in the latest Pro Tour featuring it, and it also forced players to either outpace this deck or have answers for Cori-Steel Cutter before it ended up dominating the game