Yeah I'd love that as well, it doesn't have to be complicated or even very deep. Throw random parties of mobs at you, with XP and item rewards at each "wave" you clear. Make it separate from the main game of course as to not make it exploitable to powerlevel and it would be a nice side dish.
That being said, I feel that Obsidian isn't really interested in putting anything too combat focused in the Pillars of Eternity franchise. It's a shame, I think, roleplaying is great but sometimes you just want to take a break and try a few builds to knock a few heads.