This is just an idea. What if there was an evo farming system. And you can breed them like final fantasy Chocobos. So if you breed the same type of evos, you will always get the same type. If you want celestial, you have to breed two different colors. And some heros are better at breeding than others. They can be runed to increase breeding skills. And breeding takes longer or shorter time depending on breeding skill stats. Gems can help reduce the time. This can make seemingly obsolete heroes still maintain some value.
This could be a guild function to incentivize partnership with guilds.