I am really hoping that you’re not planning to go past Champion Level 100!
Instead, what don’t you make battles that are played with your currently maxed Class to just give 1 or 2 random XP points on Classes that are not maxed out yet, below Champion Level 100?
They mentioned last Q&A that they wanted to expand hero classes and rework class events. They never mentioned specifics, but what they did say implied hero classes would have more to grind beyond what currently exists for hero classes.
It would also fit into the current coloration of kingdom stars. Stars currently go to level 20 purple (epic) where the level 20th requires level 100 hero. Whatever new system they put over hero classes likely would be used in some part for 21-30 up to 5 light blue mythic stars.
Could always specialize existing class talents. Like every 500 exp earned beyond lev 100 gives you 1 skill point that can be assigned to an existing talent to boost its effects (where those talents have concrete values. Like each skill point adds 1% to thief’s backup, or 0.5% to rising shadows, etc).