The organ progression relating to scale is certainly an interesting topic. In some other thread this was talked about: The smaller you are, the more you could rely on things like oxygen simply diffusing in your cells due to the small volume to surface ratio. As you become larger, a clearly defined circulatory system would become more and more necessary.
The issue of circulation is something which is also a reason to include scales of around 1mm. At this scale the player starting as a very simple blob of cells feels more realistic than if they started as an undefined 1cm blob. At 1cm size, most heterotrophs I know of like insects have a clearly defined respiratory system, which still isn‘t really muscular and still relies on passive diffusion, but insects at least have clearly defined cavities through which air can circulate in their interior to provide more surface for diffusion. Does this make sense? Maybe there are somewhat motile organisms of 1cm size that are much simpler of which I‘m not aware of.
Also, the issue which I raised earlier of feeding a 1cm blob which lives in a world devoid of ~1mm organisms and has to rely on consuming microbes in the micrometer size range hasn‘t been adressed so far either. Your arguments so far revolved mainly around implementability and scope which are very important things to consider. But what do you think about the realism argument? Do you guys think a very undefined 1cm blob is a more or less realistic thing to exist in the kind of world this stage would portray?