[kdtipa]

I think evolution would be harder to constrain than what you describe. To begin with, intelligence can determine how likely to survive an individual is, and give it a higher chance of reproducing than the unintelligent individuals that die off. And it's not just intelligence. Balance of fight or flight is another mental trait that can have unpredictable effects on ability to reproduce. Ability to get along with others versus solitary tendencies is important too.

And Mutations improve the ability of species to survive. When a people migrate to another region that has different animal species and different food options, the ones that can digest the available foods will survive and procreate while the ones that have difficulty digesting the food there will die off. The mutations that allow adaptation are what make life thrive. I think limiting mutations is likely to cause species to die off faster.

I don't think it's possible to control a planetary environment long term with controls set in place at one point in time and nothing after.

You mentioned keystone species... but while human influence in the real world is destroying species (like bees) that are necessary too fast for ecology to recover on its own, a planet left to go on without that kind of intervention seems to progress quite well on its own. If a keystone species is heading toward extinction, the whole system bends around them. Other species might step in to fill the role. Or species that depended on the keystone species die out too, and other species that can survive without will flourish.

I think trying to control a planetary ecology over the long term is likely to have too many variables to account for.

If I was a Forerunner looking to create a foundation for a world that is inhabitable and idyllic, I would start with a world who's sun still has billions of years left; a world that is at the beginning of its being in the inhabitable zone of its system; a world that had a notably stable orbit; and a world that already has a life supporting ecosystem that's as close to what I'm looking for to begin with.

After that, if my goal is to give my own species or humans or whatever the best chance of survival and thriving on the planet, I might avoid beneficial plagues entirely. Without the plagues to help them, they have to help themselves. The individuals capable of doing so will survive and the most survivable traits will be passed on.

If you were forcing me somehow... I might consider something that can fix birth defects in the womb. Longer life is nice I guess, but eternally young seems a bit much. Can you imagine a society where a group of people that don't die of old age get into power and then keep that power? The Super-Rich people who actually run the United States would run it forever, and things wouldn't get better. Longer life = okay. Eternal youth = bad.

Anyway, I think the most significant thing I would do to improve life for people in the long term is to leave behind a lot of information stores that have computers that can learn languages. When the evolving people find them, it will teach them things based on what they already know... but better. So, when fire starts being used to make steam engines, and they start with burning wood... the knowledge base can congratulate them, and then show them electricity and clean power sources. It would explain coal, and that using coal as a major source of steam power or electricity destroys the environment. Not looking to hide information. Just provide information enough to skip the bad steps.

Definitely provide enough information to avoid religions developing to the point of controlling people. No one needs to believe that they should donate money to a church for a place in Heaven (or whatever other invented place people might go after death). No one needs to believe that an individual happens to be chosen by a god and so that person suddenly has authority (Kings and religious leaders). I'd do everything in my power to spread knowledge so religions can't swoop in and mess things up.

Education with fact strikes me as an important thing to progress.