I see the upcoming costumes in Beta. I don’t like them and they scare me. But if we could merge duplicate heroes to get their costumes, I wouldn’t have as big a problem with it. Trading 2-3 Gravemakers for Gravemaker costume, and that being the only way to get him, sounds like a nice idea. This makes old content useful and relevant. It also gives us some reassurance that if we summon something now, it can still be useful later.
But if you do this, please do it for all the heroes and do it quickly. There’s another game I used to play–Star Wars: Force Collection. When they pulled a stunt like this (Awakening only specific older heroes so they could overpower the old meta), all the old-timers flipped tables & left. The game died just about instantly. This suggestion feels more like crisis management than putting sprinkles on a lovely cake.