One potential future for humanity I hadn't considered before was revealed recently by my Population-consumption model: population decline followed by oscillation around a new average value (what I'll call "popscillation").
I had simulated humanity's targeting of alternative worlds with populations at least as large as ours, the center of which seems to track in a predictable way with the total ecological footprint. ("Worlds" are combinations of population, consumption, and environments that people use to maximize life satisfaction, or "happiness"; the total ecological footprint, or "total footprint" is the amount of ecological resources consumed by humanity in one year.) The simulations show that over history (since 10000 B.C.) we have deviated somewhat from the direct route to the target, with that "direct route" changing over time. Every route, however, ends in a similar way, with the main difference being the size of the population.
Eventually our consumption will limit the remaining alternative worlds to those with populations no larger than ours. Since happiness depends on the ecological footprint (how much ecological resources each of us consumes per year), we've also reached a limit to how happy we can get without decreasing the population. The simulations show that we will choose to increase happiness, and with it, total footprint. Increasing total footprint reduces available resources, which decreases the largest population of the remaining worlds we can live in. That decrease drives a drop in our own population, which temporarily decreases the total footprint, allowing a slightly larger population if other species can increase theirs in the interim (creating more resources). We then increase our population, along with our footprint, which increases total footprint again (total footprint is footprint times population). This popscillation continues, with a population whose average eventually levels out at a value around 5.8 billion people, with fluctuations of tens of millions per year (assuming nothing else changes). On average, happiness is only slightly greater, people live about a decade longer, and the populations of other species fluctuate along with ours.
On our current route, we are due to begin popscillation soon, if we haven't already. Leveling out will occur over the next 50 years, unless other variables like climate change reduce the available resources further, both reducing the available population sizes and accelerating the decline. If we abandon our historical proclivities and reduce consumption enough to grow back the populations of other species to healthy levels while maintaining our current population, we risk reducing both our happiness and our life expectancy to levels not seen in a century. Such is the situation we find ourselves in on this Earth Day, according to my calculations.