If we get more demons, we need one that's basically an Imp

Imps might be a mere annoyance in most fantasy games, but they can still be deadly, especially in numbers. Plus their annoyance factor tends to be very high.

An Imp could easily fill in the "Greed" category; Tahm Kench fills in Gluttony, while Evelynn fills Lust. Nocturne, I'm guessing, fills either Wrath or Envy.

As for gameplay? Maybe take some notes from Murky in HotS. That whole "egg respawn" mechanic would be so fun to have in League, especially if this Imp doesn't give full takedown rewards in exchange for minion-level durability.