I have an idea i think sounds fun for a basic infection/immune response system.
Once you unlock a circulatory system every time you evolve thereโs a chance (which should probably depend on difficulty) of a pop-up showing up telling you your organism has been infected. What this means is 3 species of microorganism have taken residence in your bloodstream, a new tab appears in the editor, the immunology tab, showing all three species, which could be either randomly generated or randomly selected from the microbial playthrough, their names and appereance and maybe a screen showing little selection of clips of them floating around your bloodstream. If you donโt deal with the little buggers your organism will get a debuff (maybe a decrease in blood resources, or metabolism, or a percentage reduction in population, or maybe a different one for each species of bacteria; but i think they should be negligible enought that unless you are in a hard mode you could ignore them).
Fortunately by modifying some vein tissue, at the cost of MP, you get access to a primitive lymph node, and with it your first immune cell. By going to the immunology tab you can modify the cell, which will be shown under the infectious bacteria, to make it a killing machine. It should be based on the core cell like in the multicellular stage, and youโd be able to modify it by only adding external parts (cilia, flagellum, the spike of which i donโt remember the name) and the toxisome, these parts wouldnโt cost anything but youโd only be able to place two or three.
Letโs say for example you try designing a big microphage with a flagellum, it could only work if you a cell membrane that allows engulfing and if you have a high enough metabolism to have a flagellum (Resources however should not matter for the microorganism and for your cell, as the blood would provide a constanthuge amount). After you are done designing the cell the game would run a small round of simulation to show which of the microbe species it would be able to kill (a simulation might not even be necessary you could just compare the speed, damage or if the cell wall can be eaten), and maybe the screen shows random clips of the cell killing, or failing to kill, the microbe. If itโs able to kill all three great, no debuffs, if one manges to survive you only get a 1/3 of the full debuff, and so on.
The next time you evolve, however, youโll find you werenโt the only one who did so and another pop-up might appear telling you the microorganisms evolved and you are infected once again, you go to the immunology tab and find out one has evolved a flagellum and another has evolved a cellulose cell wall and your slow moving macrophage isnโt able to deal with either. This happened because the computer ran a small autoevo simulation to see if the microorganisms could evolve to survive. To deal with this, if you donโt want any debuffs, you can upgrade the primitive limph node again, giving you access to a second immune cell, which you can modify to cover the weaknesses of the other one that you can no longer modify; letโs say you make a toxin producing cell that targets the oxygen based metabolism of the two microbes, the game calculates if its able to defeat the microbes and gives you the debuffs accordingly.
After evolving a third time you can, if necessary, modify the lynph node a third time to unlock the final immune cell, lets say you make another toxic cell that targets anerobic metabolism or a ram-like cell with the spike and a fluid membrane for speed. Once again the game does its necessary calculations, shows you your cells in action and you move on.
Finally if somehow after all this a microorganism evolves who is able to escape all three of your cells (maybe the higher the difficulty the higher the rate of mutation) you can upgrade the lymph node to a kind of thymus for the final time and make it so your immune cells also go through auto-evo to always be able to kill the infectious cells.