A flatscreen mode similar to CitraVR might be better than the current Seated mode for keyboard and mouse players. You'd still get the 3d effect, but look at a screen instead, reducing the motion sickness you'd get from trying to play with KB/M and a headset.
A similar example of this was what Minecraft: Gear VR Edition did.
No background would actually be needed for people to be able to play it, obviously, but it did go a long way towards the polish you got from GearVR Edition.

yeah, I lready thought about a theater mode a while ago, not a priority though.