Processes
You would start with basic processes, like anaerobic respiration, protein making, etc. You would be able to unlock new processes, by evolving them somehow, or conjugating with another cell. Conjugation would not be possible with a nucleus, or with a too distantly related species. This would allow you to unlock all processes that the other cell has, and that cell gets all of your processes.
Endosymbiosis
You would be able to absorp cells, and use their processes. You could change how effective a cell is at something, but increasing one process reduces the others. Endosymbiosis would allows you to specialise different organelles for different processes. You could also ‘conjugate’ with endosymbionts. If you got endosymbioted, you could also still play as the cell that absorped you (Because it would still have your DNA)
Nucleus
You could evolve a nucleus by creating a membrane around the cell’s DNA and then making it a nucleus (Or however the normal nucleus-making way will be). However, you could also get a nucleus by entering a large prokaryote, destroying its DNA, taking control, and becoming the nucleus.