I’ve liked this idea ever since ~sciocont suggested it many years ago: calcium-producing microbes.

The idea is simple: give your cell organelles for synthesizing calcium carbonate from compounds in the environment and a modified agent secretor which spills the result into the tidepool on the player’s command. The catch would be that it solidifies in the environment and sticks to existing solid calcium carbonate blocks, giving the player the ability to build their own calcium-based microscopic structures.

The possibilities are endless. Cellular shells, enclosures to herd and protect other cells from predators (maybe if they produce a compound you want), abstract art. Providing movement control is fine enough to give the player creative freedom, I think this would be a really cool strategy for them to have.