I would think if dev set different arena entrances with different rules would be easiest rework. This could use the existed Arena framework but make it multiple mini-game which have different taste for players.
Some of rules for different arena entrances:
Remove Hero from Arena (Because weapon is OP),
Allow select banner.
Unlock all troop traits (all troop have traites unlock no matter you have upgrade their trait or not)
Adding a legendary troop option.
This would make game back to its fun mode.
And also make 200+ gem to buy Arena troop option reasonable. ( Who will even bother use this option…)