With my previous engines they tended to move more and more towards using sprites as the main object representation, much like most ray casters and Doom-like engines.
One of my previous projects contained polygon "sprites", literally in stead of sprites being draw, polygons are inserted into the sprite rendering code.
The XI Cast engine will take the direction of using both sprites and models, models will be for the more advanced users and sprites can be inserted by anyone, however, with the new 3D camera there is a small change with how sprites are drawn.
This image shows the alpha testing sprite to show that transparency has been re-added, however notice that a couple of pixels on the rainbow gradient to the left are shifted. This is because sprites are now drawn as polygons themselves, giving them the perspective shift when you look up and down.
This image of the sprite's plane shows exactly the effect.
The option of locking the camera's up/down view is there if you want to have it for your project, that would preserve the "2D" look of sprites, like in Doom and Wolfenstein 3D.