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
He commands the powers of the instant like no other, capable of summoning forth fearsome foes to do his bidding with supernatural speed, sending these minions to decimate the opponent in one fell swoop
The card is exceptionally slow, but in decks that care about +1/+1 counters, this will create a lot of value
1 Rapacious Dragon
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
This is a pretty solid list