Supergirl has always been filled with a wide variety of names from Superman stories, ranging from comparatively small DC characters like Cat Grant to the Man of Steel himself. The latest episode gave a nod to a classic DC bad guy who qualifies as one of the biggest villains in Superman's entire comic history. The episode featured alien gladiator Draaga, who spent most of his screentime trying to kill other aliens for cash, but his significance was proven to be far grander due to his traditional connection to the supervillain known as Mongul.
Nobody on Supergirl name-dropped Mongul himself, which was probably a smart move. Viewers who have a knowledge of comic history might have forgotten everything else that happened in the episode if somebody actually mentioned the legendary bad guy. Instead, Mon-El casually informed Kara that he'd once seen Draaga fight while on a visit to Warworld. Mon-El didn't go into any details about Warworld, but it is best known in DC lore as the roaming artificial planet controlled by the super strong and super brutal Mongul.
Mongul made his DC debut in 1980 when an exiled Superman ended up in the Warworld gladiator games. ("The first rule of Warworld gladiator games...") Superman made an enemy of Mongul when he defeated Draaga but refused to finish the fight by killing him. Mongul and Superman became mortal enemies thereafter, with Mongul hunting Superman throughout space and eventually arriving on Earth to destroy Coast City, which does exist in The CW's DC universe. A big bad with a big bad artificial planet and army of slaves is a threat in any situation.
Basically, Kara is in for a lot of trouble if Supergirl ever goes so far as to adapt a version of Mongul for her to fight. It's not inconceivable that the huge Superman villain could be borrowed for Supergirl to battle; Season 2 managed to turn the Man of Steel into a side character to keep the focus on Kara. Mongul could turn up as a villain just for Supergirl without any initial connection to Superman.
Of course, it's also possible that the Supergirl introduction of Draaga and mention of Warworld were only meant as Easter eggs for comic fans (such as Coast City). Mongul may never appear on The CW in any way, shape, or form, even if the show someday reveals that he's out among the stars with his roaming planet. Whether he shows up or not, the tease was a fun addition to the latest episode of Supergirl. We'll have to wait and see whether or not the show does any more to introduce Mongul into its universe. Mon-El visited Warworld himself, and Mon-El doesn't seem to be going anywhere any time soon, so there's a chance.