So, what if there was a sim (Can’t call it a game, since you only interact once or twice). You have two single-celled organisms. You get to decide the traits of one of those single celled organisms. The other player decides the other. They then wander around, eating, being eaten, and slowly getting mutations. Eventually, the two organisms will start to compete, and then one will live or die. I got the idea from this game. What do you guys think? The mutations could be appendages, and stuff like that. There could be stuff that they could only reach if they mutated right, and then that stuff could mutate them for the good or worse.
What do you guys think? I probably won’t get to this for a while if I decide to make it. Feel free to steal my idea!
This would be an awesome idea but since you only interact with the game a few times, it would have to be extremely visually interesting. Hope you can get to it
I had this idea to (but with more interaction), but it wil be really, really hard to implement this.
Every mutation needs to affect the creature sumehow, and mutations also will be affected by the behaviour.
Groups of spieces also follow diffrent mutations because other enviourments and the random factor.
Try to think how to render or calculate all these factors, its sooo much 
I was tempted to make a web browser game where a person makes a program that determines how their plant grows. Grasslands online just another thing that will probably never happen.
The hardest part in most cases is finding interesting attributes for the player to mess with, stimulus that the players can easily understand, and strategies that are not utterly dominating.
If you want to get ideas look at how bacteria in the gut works, deep sea ecosystems(especially whale caucuses), and maybe some molds/mosses.
If you want something more game like maybe creeper world could provide some interesting ideas. http://www.youtube.com/watch?v=AKndk7Msgc8
edit: missed a negative
“It’s like watching grass grow!” is probably not the review blurb one normally aims for, no 
This very much reminds me of spore.
Spore had plenty of good ideas, it simply failed to present them in a compelling manner.
Ironically, the simplest phase of the Spore game, the cellular minigame, was the best one (if only because the simplicity matched the tone and feel of that part of the game perfectly).
Problem with this sort of ideas is making them fun. Having players take turns at what essentially is a slightly more complex Game of Life isn’t the pinnacle of entertainment.
What I personally would’ve loved to see with Spore is a series of games, with each game focusing on a single phase exclusively, and then providing the option to carry the results to the next “phase” should you choose to.
Oh darn… No, no, no, no, I have to focus on my current project… BRAIN! STOP HAVING IDEAS! :o
I’ve tried that sort of thing a couple times, once with a WC3 Map and once for a Ludum Dare. My idea was that you go gather various creatures that have advantageous stats, then you breed them. Through mutations, you can evolve the best creatures possible.
The big glaring problem in both implementations was feeling like you had no control. In the former, it was extremely difficult to even convey what all the stats were (due to WC3 limitations), and in the latter it felt very random and it wasn’t clear the best way to do things. Certainly either problem could theoretically be fixed, but the common theme is that it’s very difficult to communicate how to do what you should.
Your idea of just a couple interactions might be pretty gold. I’ve had a rather great time with this genetic cars “game” even though I have as close to zero control as possible. I sent the same seed out to several people at work and we “competed” over who reached the highest distance.
Anyway I think there’s vast potential in the evolution space, but it’s a very very difficult balancing act.
I had a similar idea that would be part of a bigger game. I was thinking of a platformer RPG type game with animals that compete with each other and eventually grow. They would all start off with different traits, behaviors and “stats”.