A new Legends of Runeterra cinematic seems to show Irelia, Malphite, and Zilean as the next Champions to join the game.
The trailer shows Zilean watching over various events, and later brings in Irelia and Malphite.
"Runeterra is in turmoil! The Void threatens invasion and Noxian warships set sail for Ionia’s shores"
With the expansion being titled Guardians of the Ancient, the three Champions featured would make sense to be released. Zilean is seen watching over different regions of Runeterra, before rewinding time to help create more positive events.
Irelia is seen as a guardian of Ionia, and in this cinematic she finds herself fending off Noxus invaders. Malphite is part of the Monolith, which was said to hold back the Void. When the Monolith broke, Malphite was all that was left; but his duty stays the same. All three fit a 'guardian' trope.
Of course, until they're officially announced we do not know for certain if these Champions will appear in this expansion.
The Guardians of Runeterra is due to release May 5th, with more content to come.