This would be a great idea in my opinion.
While the main focus of the game should probably remain the PvP aspect, I think some more PvE elements after having finished the campaign mode would be kinda cool.
I think this needs a few cooldown mechanics in addition to the cards cooldown. Like maybe only 1 battle entry per day. Between Frontier, modern and survival, it's already hard to keep up without spending half of the day playing splinterlands. So I think when adding another "game-mode", it should not take more than a few minutes per day to compete.