I know you can make equipment plans for heroes but the adventurer character specifically really needs to be able to have three different sets when switching since it's the only hero who can switch between elements.
For example, I switched from fire to ice for one of the dungeons. It automatically unequipped my positive fire rune since it doesn't apply to the ice element. But then when I switch back to fire, the rune stays unequipped, by having these different sets it would allow you to switch without having to equip stuff again.
Frankly this specifically is very bad game design. Either remove the positive and negative runes requirement to be equipped on a specific element or make an improvement to the plan system/have the adventurer specifically be able switch sets with ease.