Compose a piece for string quartet.
You will establish a “cantus firmus.” It should consist of a well-know (ideally public domain) melody presented here in, at minimum, double augmentation. It could be presented, and remain, in the cello part, though it could be distributed between the other parts at your discretion.
The other voices should be, in contrast, highly active. Imitative counterpoint is an interesting possibilty, though you might also explore other options (different “strands” of ostinatos, for example).