There are two parts to this.
First, before the big changes to the lore, there were already six what I call Outsiders: Nasus, Renekton, Morgana, Kayle, Taric and Malphite. The latter four have yet to receive a change to their lore, but Nasus and Renekton were changed from godlike entities to Ascendants, former humans who performed a ritual to turn themselves into godlike entities; Azir and Xerath would later join their ranks. Back during the early generation of League, the idea of the League was to summon allies from distant worlds (hence the six Outsiders), but that concept was quickly dropped with Kayle's lore, where she offered her services to the Institute of War in exchange that they stopped doing that shit.
Second, I'm not sure it would fit in with the theme and tone of the game, which varies from a pseudo-medieval setting combined with Final Fantasy VI-esque magitek and borderline Dungeon Punk. We already have offworlders; the four remaining outsiders besides, we have whatever the hell Mordekaiser and Hecarim are, along with the Void creatures, Nocturne and now Bard.