Due to the extremely small numbers of sponges that have been homeopathically proven, any sponge provings would be very useful to the homeopathic world.
Due to the fact that two Desmosponges have been proven, more extensive provings of both Hexactinellid and Calcareous sponges are strongly recommended. One remedy, Clathrina clathrus has already been made into a remedy by Remedia, and so would be an excellent candidate for provings.
In terms of sponges which may be particularily fascinating to prove, I would like to suggest both the Carnivorous Sponges and Photosynthetic sponges.
Carniverous sponges are desmosponges of the family Cladorhizidae which passively capture small organisms and actively digest them extracellularily. I cannot even speculate on the homeopathic themes this sponge would have, but would be fascinated to see the results.
A second group of sponges that would be fascinating to prove are the photosynthetic sponges. A diverse group of sponges have evolved symbiotic relationships with photosynthetic algae or cyanobacteria (precursors of photosynthetic organelles of plants) bringing them very close to plants in their life modes[i]. This crossover, in the context of a layer one remedy family would be fascinating to observe. These sponges are particularly abundant in Australia, so I do hope my Australian colleagues will take up a proving of these magnificent animals. One genus suggested would be the blue sponges Collospongia.
Lastly, while not strictly a sponge, another remedy which would be most illuminating is the choanoflagellates. This group of organisms represents the boundary between cells that live as individuals, and cells that begin to work together in organisms. They lie at a crucial point in evolutionary history, and thus would have great significance within homeopathy. A proving of a pure culture of any choanoflagellate species would be most enlightening.
[i] Lemloh, M. et al. Diversity and abundance of photosynthetic sponges in temperate Western Australia. MC Ecology 2009, 9:4.