Simply, what allusions would you next want to see in the show? Bear in mind, that anything modern (read as: anything from the past 50 years or so) would likely not be in the show at least until "the end is nigh" (something like the 20th volume if we're being optimistic). So, while choices like Marvel or DC comics, Star Wars, video games, etc. may be obvious, they're not really applicable and should be left for later discussion (preferrably, something like 5 years from now).
Now, that being said, RT has made room in the show for dual-allusions. This is made obvious by Team SSSN. Their team allusion is Journey to the West (3 demons/gods and one monk). Sun is Sun Wukong from the same book, but Neptune is the Greek God, and Sage is Aesop (also arguably of Greek descent). It is therefore possible that a Team Allusion could be based on X-Men or Star Wars while the individuals in that team are based on older archetypes (an Avengers motif could have Frankenstein as Hulk, an archangel as Vision, Lancelot as Iron Man, etc.) so, all hope is not lost.
This is a topic which has probably already been discussed at length time and again. But dredging a dry well can sometimes turn up some interesting treasures. Let's see what this gets us.