I have to disagree. I think that there is a lot that can be done with relatively simple additions to construct. It won't be crysis, but with some basic controls I think some really awesome stuff could be made.
My list for what would be needed:
3D model object
play animations (no need for inverse kinematics, ragdolls or any of that complex stuff)
3d lighting (again, basic stuff)
camera controls (set xyz, etc)
set sprite's Z position
perspective rendering (<--As far as I know, construct renders things with an isometric (no perspective) camera.)
If we were able to set that to a normal perspective-based camera (perhaps on a per-layer basis?) and had the abilities listed above, we could make all sorts of awesome 3d games.
A couple 3d behaviors might be nice too, like 3d bullet. I'm not a 3d programmer, so forgive me if I underestimate the task of what I've listed, but as far as I know those would be relatively simple as far as 3d goes.