These are my personal ideas for how I hope things will develop.

- Portal vis system.  Have distinct borders between rooms, such that
the door can close and completely eliminate the blocked off room for
vis tests.

- Remappable portals.  Exension of above, to allow changing which room
is on the far side of a portal.  (assuming doors are exact same size)

- Room duplication.  Since you can change what door leads to what
room, add the ability to copy the room.  This would specifically mean
reusing the static (including visibility and texture) data between
each instance, while the lightmaps and such info would be given
independent data for each instance.

- The above items would give you the ability to have randomly
generated maps.  You'd have a base map which would have one of each
room you wanted available, then you'd pick between them, connecting
portals and duplicating rooms as necesary.  You also could (would have
to) modify the multiple-texture mechanism currently used for button
animations to allow different bases in TF have different coloured
walls.

- Variable-detail models.  Have 3 or 4 different versions of the same
model, each with a different amount of detail/polygons.  You could
then make it switch between them either based on range, detail levels
chosen, or preferably a combination of both.

- Skeletal model animations.  Nothing special here.  Just seems alot
more suitable than vertex animations.

- "Inverse kinematics".  Or whatever you want to call it.  Basically
just stuff about moving legs and such in a realistic way.

- "Skin Bones".  Basically extra bones added to the skin of
high-detail models, that attach to the underlying bones, used so the
skin stretches in a realistic way when you move.

- Variable-detail walls.  Same thing as a variable detail model, but
you don't need a skeleton.  It'd be fixed to a specific place on the
map/wall.  Used for extra bits of detail that aren't necesary for
using the map, like having a high-detail statue vs a low detail one.