The standard zbuffer which we're using now renders from front-to-back for pixels, such that if two polygons intersect it will show the intersection properly and no time is wasted drawing pixels that are blocked by other polygons.
The zbuffer has downsides - beside slowing down the GPU, it's not always accurate (especially if the range of vision is deep), it doesn't allow proper display of translucent polygons, and sees all polygons as a random "soup" with no object-level abstraction.
PySoy inherited intra-object BSP tree calculations from Soya allowing large objects (such as a whole level as one entity) to use BSP tree optimisation. This can easily be extended to all entities such that every object can have unseen polygons pruned and the faces rendered in the proper order.
If objects cannot intersect with a physics model which forbids it then objects can be sorted by a BSP-tree dynamically, then each object can be rendered individually by it's precomputed order. Further optimisations can be made to only recalculate the object-level BSP-tree when objects have moved sufficiently to justify it (with a dynamic accuracy based on CPU load).
Just some ideas to chew on, nothing that's going to get into 1.0...