Starting somewhere around 12 star titans, it is near-imperative to have an elemental defense down hero in each color. Some alliances have even made it a requirement to join that one has these 5 holy-grail heroes. (Or 5 of 6, since Arthur and Frida both do blue DD). And yet some of these (Evelyn/Frida) are only available at most once a year at Atlantis, and the others (Falcon, Panther, Jackal, Arthur) are available approximately twice a year. It would be highly useful to be able to pull this sort of hero in any month, and not just occasionally. Some of the S3 heroes will help here, but the 3-star DD in blue is not going to help the specific problem, as a 3-star would die if a 13* Titan looks at it cross-eyed.
I think the costumes offer an “easy” way to do this. If one 5* hero of each color has a costume that provides an elemental DD, we would have (1) more people pulling at the costume chamber and (2) more elemental DD available for fighting titans.
Specifically, I’d suggest:
Purple: Domitia’s costume could cleanse self and DD target and neighbors. She continues damaging only one.
Red: Marjana and Azlar are pretty much defined by DoT, so Khagan gets it here. It probably just replaces his mana boost.
Yellow: Should probably go to Leonidas. You could play with effects a bit here.
Green: Either Kadilen or Elkanen should get it, probably Kadilen, so as to be “different” from Evelyn
Blue: Thorne should probably get it, probably keeping his splash damage to differentiate from Arthur…