Be creative. What sounds fun to you. I personally want rock eating microbes and I want them to be able to tunnel.
I also want iron respirating microbes who eat little chunks of iron floating around.
Be sure to back it up with at least a modicum of science. But this is a creative excersise so meh. Gameplay is important.
For example I want to add lysosomes but it would just be a way for good players to challenge themselves eg. You place them now you have to do mitosis in X amount of time or have apostasis. So it would be s totally optional way of challenging yourself.