I was thinking about how cancer could be included into the game in a fun way. Cancer is important for pretty much all multicellular life, particularly for animals. It would therefore be nice to find a place for it in the game.
Some Cancer Science:
Cancer is expected to become more of a problem when an organism has a larger body size. This is because larger organisms consists of more cells, increasing the chance that any of these cells becomes a cancer cell. Larger breeds of dogs for example are more likely to get cancer than smaller breeds of dogs. However, species can vary in their cancer resistance. A whale for example is actually less likely to get cancer than a human is, because whales have better cancer resistance.
I think cancer resistence could be used to restrict the number of cells an organism can have. If you want to have more cells, you need to get better cancer resistence. This could be a hard limit on how many cells are allowed.
Alternatively, one could let the player add more cells, but introduce a cost to cancer. In nature cancer might simply be expected to kill the organism. However, it isn’t much fun to randomly die whenever one of your cells becomes a cancer. Maybe a cancer cell could instead start a countdown. If an organism gets cancer very early in life, the player could simply restart, but if cancer happens late in life, the player still has time to finish reproduction.