With that strong a roster and that many 60s already, your next one would depend on which part of the game you want progress in... there are a bunch of 50s who would be worth 6*, but which one to choose depends on your priorities. For instance, I see a Maneater, you could take him to 6* if you want to go for an Unkillable Clan Boss team (and if you have a Painkeeper, or the other options to combine with ME for a UK team). But if that's not your priority now, maybe you'd prefer somebody else, like maybe Umbral Enchantress for arena, or Sicia Flametongue for Spider, or...