Besides community-made formats like PreFIRE, Classic Legacy, and 2015 Modern, one way I like to play with uncompetitive cards and strategies is through Spice Paradise, the Premodern webcam league I organize that prohibits top tier strategies
In fact, Primeval Titan has been the face behind some of the most degenerate decks in Modern, starting with the Cloudpost deck that was prevalent in the initial days of the format, to the Red-Green Valakut deck, and then the Amulet Titan deck both in its degenerate form with Summer Bloom, and the more toned down version we see today with Dryad of the Ilysian Grove and Urzas Saga
One of the companions available in Baldur's Gate 3, Karlach (along with all the other companions) was given a creature card and even had a whole Secret Lair based around her
The Fourth Doctor The Fourth Doctor provides card advantage by letting you see and play cards from the top of your library
This is a pretty solid list