I'm not saying the devs should make a plugin, but I do agree with Arima... If a a 3D model plugin were ever to be made, it wouldn't need to have collision capabilities. Construct already does collisions just fine with other objects.
A model could be used in lieu of a sprite to make games like Tomba or the new Bionic Commando, where the gameplay is purely 2D. The current method that people use to check collisions - with collision detector sprites - could be used in exactly the same way, but instead of an animated sprite, you set a model on top of your detector. You wouldn't need to have collision capabilities for the model at all.
The only things that would really be needed would be the ability to rotate, scale, and translate the model (like you can with the 3D Box), and to change animations. The rest of it, collisions and whatnot, can be done in Construct in whatever way the designer chooses.
Anyway, it would be a nice perk... perhaps some clever plugin maker will make a clever plugin for it someday. That's what open source is about