Would it be possible for a cilliate, such as Stentor, to evolve into a large multicellular organism similar in form to the original unicellular body-plan?
The constituent organism of a volvox colony are not ciliates, but photosynthetic algae with flagellae. Their photosynthetic lifestyle lends itself to scaling up; more surface areas and predators the size of the individual component cells cannot fit the colony in its mouth.
Ciliates are non-photosynthetic which makes it trickier, but I can imagine a similar scaleup for those that feed with a sort of ram-scoop mouth. Volvox have just 2 cell types - the reproducers in the middle and the workers on the outside but a colonial ciliate with the same body plan would probably need both of those and a third type dedicated to enclosing food items in enormous 1-cell vacuoles.