Intergrated Particle effects/physics or animated particles.
I Know this can be done through the animation to some extent but a major drawback of clara.io is the fact that one simply cannot utilize particle effects on this software. The following could include Decal based smoke or proper 3d shape generated rain particles.
There is a lot of flexability around this but it seems clara.io is only useful for modeling, not the animation/utilization of said models. Particles would give models some purpose and give reason to make them in the first place.