Look at who they've cast in the past... Nicholson - Oscar Winner, Ledger - Oscar nominated, Leto - Oscar Winner, Phoenix - Nominated, guessing they will find an actor who has some awards pedigree/artistic credibility but isn't already a superstar, seems to be the pattern except for Nicholson who was a huge name upon being cast.
If Joker is going to show up in The Batman sequel, I would pay attention to the supporting actor/lead actor nominations over the next few years as they may pop up there beforehand!