My suggestion for Niche is the ability to breed specifically for longer or shorter aged creatures, and this as a result would have an effect on a player's strategy. Each lifespan would have different advantages and disadvantages.
Genes effect many things in our lives, including how long we live. Just look at the amazing variance in lifespans among different mammals, even those of similar size (naked mole-rats can live 30 years as compared to rats that live only 2 to 3), and even within the same species (the variance of lifespan in dog breeds). Different lifespans work better for different lifestyles in nature, and I think they could work for different play styles in Niche as well.
One could breed for creatures that lived for more days, but with the pay-off being lower fertility and longer time spent maturing, much like we see in real-world animals like elephants, and whales. Longer lifespans would be perfect for a group of nichlings that like to migrate, giving the player time to take a handful of individuals to several different islands, before settling down to breed, which is a strategy I personally like to use. The problem with this strategy currently is that it feels like a race to balance breeding and traveling before your nichlings pass away. Nichlings that lived longer would make for a slower paced gameplay, with baby nichlings being vulnerable for longer, breedings being more spaced out, and the population's genes being slow to change.
You could also breed for shorter lifespans, which would be a much faster paced option to play. In areas that have many resources this would be a great adaptation, allowing the nichlings to reproduce and mature much quicker. As a result you can breed in new mutations a lot more quickly, much like with mice and rabbits. But it comes with it's own drawbacks as it could take several generations of nichlings to reach a new island, not to mention if food supplies grow low, the nichlings with fewer days to spare would be at a much greater risk.
Then of course, you'd have the middle ground, which I imagine would be the same as it is now. Or conversely you would have a mix of differently aging creatures within one tribe, which could also lead to some interesting strategy and experimentation to see which lifespan suits which niche best.
I feel like this should all be tied into the fertility gene somehow, as this seems to be the case in real life as well. Longer lived animals generally reproduce much slower, while short lived animals reproduce rapidly. As an example, perhaps double High Fertility equals a shorter-lived nichling: double Low Fertility equals a longer lived nichling; and double Medium Fertility equals medium-lived nichlings. And all other fertility combinations, including no fertility would likely just result in medium-lived nichlings.
You could also simply have it so that longer lived nichlings have to wait longer between breedings, if it works better as a separate gene. Short-lived can hop off the nest and breed straightaway, but the medium lived must wait a day or two, and the long-lived, a few days more. And same for the baby nichlings maturing. Short-lived nichlings will mature much faster, medium will stay about the same, and long-lived nichlings will take longer to mature.
Now, I know nothing about game design, so it could possibly prove too complex to balance something like this, but I figured it was worth suggesting, as I've been kicking it around in my head for a while. The thing I like most about Niche is getting to approach each tribe with a different strategy depending on the environment and the genes provided you at the start, and I think this could add another really interesting element to that.