I disagree. If they spent time and money they got through Kickstarter to work on a mode that was never mentioned, it would be inevitably taking focus, time, and resources away from the single-player and co-op.
Those are the core focus and should get priority. An arena mode was never mentioned in any of the information for the game, so if Larian thinks it's something they want to add, it should be as DLC (free or otherwise) after Original Sin goes gold.
