The way I imagine it is you still do all your questing and build-making the normal way, but then there is some place that you can go (or maybe a covenant, whatever) where you leave the normal 3D world and enter the arcade fighter world.
All of your equips remain, and the controls are identical (except the left stick becomes 2D movement, I don't even know what the right stick should do) and the movesets for your weapons are altered for 2D, but mostly remain the same. Spells and miracles function the same way as ever. Backstabs would be hard but possible if you got behind your opponent. My imagination is getting away from me. I'm gonna stop there.
Maybe it can be a 2.5D fighter where you can side-step projectiles. I wouldn't want to go against anyone using firestorm, gravelord sword dance or WOTG though--you might get cornered and combo'ed/juggled to death.
If it were like super smash brothers, you could just dodge in place or be invincible throughout your entire roll. This just made me think of super smash brothers with hp, parrying/backstabbing, and unique customized characters.