Well, Arcade mode offers more than just skills. There are town upgrades that affect gameplay and will keep you busy for a while. Also, current Arcade Mode has like 2/3 planned floors (it's supposed to have 24 IIRC).
If new skills would ever be added, I think it would be cool if they were secret skills. Nothing could be more rewarding than finding a scroll with completely new skill, maybe from some new elemental. Or an upgrade to existing skills, something like Super Skillpoint, that adds a new charge level increasing skill's power drastically. Of course if these were to be added, there should be choice on what new skill you want to unlock or what skill to upgrade, because scrolls with fixed skills force a particular gameplay.