So, today's thought experiment: How can you make the game Spore fun?
If you've played Spore, you've probably already thought about it. Very cool content creation tools yet to be matched by anyone, but no gameplay. None. The designs don't matter, the game phases have no complexity and almost no meaningful play. It wasn't even made into a casual game: it was made into a non-game.
So the question is: how do you make Spore fun?
I can see why they removed making the designs matter, as the freeform design makes it a really concentrated skill challenge. Not only would players poor at design have a rough time of it, but also any player that didn't want to spend a week tweaking their character design would come out worse than the "standard". Similarly, there is both a learning curve and a strict optimal set of designs when things like complex gaits are involved.
When you stop and think it through, you can clearly see that free design is a very high-skill kind of play. While I would love that, it would make your first playthrough of the game your worst, which is generally a bad idea.
What solutions can you come up with?
My solution is to make it more like real evolution. That is, once you make a change, you can't ever change back. All you can do is keep building off the choice you made.
For example, you start off as a simple worm-like water creature with a spine and a mouth. You can, if you like, turn one of your vertebrae into joint-bearing vertebrae, and you can steadily refine that into fins, arms, legs, wings, whatever you like. But you can't revert it back to a raw vertebrae, or move that formation to a new vertebrae.
Of course, you can extensively reshape everything within those constraints.
There are many things you might want to assign to a vertebrae. Ribbed chambers are useful, but they do strictly limit the size of the organs within. Pouch chambers are great if you want room to grow, but there's no structural support or armor. Leaving the vertebrae raw could be useful down the line: you'll never manifest a full set of arms out of nothing, but you can start the painstaking process later if you decide you need more arms.
The opposite happens, too: you can wither away the limbs or chambers you've specialized into until you can't even see them (although generally you'd just reduce them to structural vestiges rather than spend the evo points to completely diminish them). While they no longer have the function they would have had, the vertebrae is freed to maneuver as if it were raw and no real energy is wasted on generating them. And, of course, later you can evolve them back into existence step by step by step...
The same kind of sequence series thing works in terms of neural setup - evolving sensory organs and cogitation in the same way you'd evolve limbs and chambers. Organs, too, within the chambers...
All in all, this setup should make designing easy to get into right at the beginning, and let you grow into the more complex designing features iteration by iteration, never having to spend too long designing at any one time.
This one's just a simple setup, but it could theoretically possible to be a lot more complex and free while you do this. But it'd take a long time to explain, so this'll work for now.
Does it sound fun? What would you do?