This is very good question.
Main Concept
So my main concept focus on lifeforms evolution.
As the core base is, where lifeforms bringing new offspring, then these mutate, affecting their own traits and cells.
Having more cells, allow to have more traits, or strengthening existing one.
Some may be good for specific conditions, other don’t.
I.e. chloroplast cells as part of the plant traits, allow to gain energy from sunlight. Energy gain is low, but is pretty much free, like solar panels. However day / night cycles have an effect on produced energy level.
While muscles cells allow to move, which cost more energy and along with herbivore traits, allow to hunt lifeforms with plant traits. In long run, much more efficient for gaining food and energy production. There is more into this mechanics, but no need for details at this point. However, these are in game already and are to stay.
Also, there is faction mechanics, where lifeforms from one faction, will be more hostile, against other faction and can compete for a space and domination. Could say, evolution race has started. 
While game can run on its own as simulation, I am introducing more mechanics, which allow mutate new offspring, in more desired direction.
Building Cellular Lifeforms
Primarily I had lifeforms represented as cells, which are voxels. So each cells, or building block, adds new functionality. Potentially “building” large species, like trees, or elephants 
Sure they won’t have moving lengs, or trunk, but can resemble of such. Of course, game simulation (AI) may be looking for more optimal cellular evolution and its structure. And I would like keep that mechanics somechow. At least to certain extent… Even with new proposed L-System, I can check for position of plant components, to find out, if has benefits, for specific environments. I.e. chloroplasts / leafs lower to the ground, will gain less energy, than up high, or in middle of the lifeforms (useless and cost energy to breed such offspring as well.).
Problem with building by hand mechanics is, It seems, or feel too tedious, to modify larger cellular lifeforms.
Not sure if it will be fun, while game is running and things are changing around. I would rather have something, which allow player to guide evolution direction of desired lifeforms or species, than getting focus on to micro level.
I think there will be plenty to do already anyway.
I mean, I loved building in minecraft or From The Depths, and making stuff move and fight as well (as meny do too). That is micro management sure. But main difference here is, I am dealing in EcoPico with potential RTS alike situation. What if I would like to have implemented multiplier in (far) future? (rhetorical question, thinking aloud).
This is why I thought, bringing something like L-System, to give more organic behavior, could be potentially interesting and hopefully fun to play with, if executed well (yeah, define well if no one done that? 
).
I am hoping to find some sensible and interesting solution.
Visual Aspect
Saying all that, as in current mechanics and new propose L-System mechanics, I want visual parts to have more meaning, than just visual. Like specific cells appearance, carry also their specific functionality.
I do realize, that going from cellular level, to more macro level, like plants and trees, may break my some initial concepts. But I am keen on exploring options.I just hope, not to diverge too far from main scope. But on other hand, I want make game more appealing to potential audience.
Consider just pure plant tree. Here for example, starting with single branch lifeform, as if first cell. Gaining certain traits over evolved generations, would allow to grow leafs, or rather first leaf (maybe). Other traits would allow to make branch bigger, storing more energy etc. Few generations later, new branch would appear. Our tree now having two branches or so. Then can focus more on leafs evolution. Should be going higher, or wider, etc.
Now you look at lifeform and can judge briefly, what is capable of and what some of none hidden traits may carry.
Player then can focus, on evolving specific strain of lifeforms.
I hope above makes sense.
Any thoughts, on new proposed system? Again I would like have trees more simplified, than per my vid from preview post.
Also, I would love abuse ability of DOTS.
Ehh, I think I made myself a bit carry on
But that helps me think about core mechanics and where I am heading vs where I want to go.
