Reminds be a little of John Nash's boardgame where players need to connect a line across a chasm; combined with a kind of block-tower builder game (but with irregular pieces).
If I could make a suggestion;
it seems that the endless wave of new cells adds a little monotony without any intentional pause between a 'set' of levels. if the conditions between each level set are different, then each level set can be 'replayed' with the novelty it provides. Perhaps playing to unlock an 'endless mode' for the challenge that set of levels?
Some thoughts:
Not just suspending the cells in water, but also perhaps suspending them in alcohol as well (which is less dense than water)...
..while having a greater variety of cells -- some of which only suspend in water, and some of which only suspend in alcohol? Then you can have a 2 tier system of flotation to simply an overly complex grid layout.
The only other cell shapes I could think of would be Semicircle, or Rhombus/Diamond. 5 shapes would have to be the maximum if you were to add complexity.
Player options might include being able to select the solution mix to add to the experiment, thereby intentionally choosing the probability of what kinds of cells will be dispensed for the next 20 or 50 turns?
Your game is really good already. Just offering suggestions if anything jumps out at you.