Featured Atlantis heroes as well as featured old HoTM have a 0.25% chance to be drawn each. Non-featured Atlantis heroes have a drawing chance well below 0.1 % (don’t remember the appearance rate, however, it has to be divided by 8 if you try get Tarlak when he is not featured…).
As @MysterySpin has already said, the odds for summing a 4* costume are 26.5%. Depending on when Wu’s costume will be introduced, the odds of getting his costume will be at least 1.2% … every month.
Any F2P or C2P and even most P2W players need a lot of luck to still get Miki or Tarlak (Ranvir is basically a 5* Wu). If there were a new HoTM with an effect similar to Miki/Tarlak, also lots of players will NOT get that hero. As we already have two Legendaries, I’d be really happy to see a 4* hero with a similar effect, be it the costume of an existing hero, be it a S3 Epic hero. This would give all players a decent chance to get that hero–maybe not the first or second try, but they would be able to try for that hero every month for a >1% chance. Also, that would add some more variation to 4* weekly raid tournaments