One thing that jumps to my mind is that it would be ideal to have a good representative starting point of the Multicellular Stage developers can jump to. We thankfully have the free build tool already, but the cells there start with hydrogenosomes, which does not represent the energy balance of the average playthrough multicellular starting point (aerobic respiration).
So if it can be made so that the multicellular free build tool starts with a preset amount of oxygen in patches, I think that would be very beneficial going forward. Designers can obviously have a save ready for testing something multicellular, but that would be very world specific.
Beyond that - there are obviously larger design questions to be had, but I think we can make sure that the multicellular isn’t painful right now. For example, two things that jump to my mind are making sure that colonies aren’t moving like molasses, and making sure timed reproduction isn’t way too long. Targeting a 2-3 minute lifespan I think would be ideal.