who is next

None that we know of. We only have one data point here: humans are the only species that, as far as we know, ever have developed intelligence on this planet. We do not know what conditions are required for such development, what species can in principle develop it, etc.

Perhaps some bird species becomes intelligent eventually, if we are wiped out. Or some insect species. Could be anything. By various studies, dolphins seem to be the closest to having intelligence among all the species we have investigated, so they are a good potential candidate - and since they live in the water, a civilisation they would build would likely be very different from ours.

@maxx

By "intelligence" here I imply the ability to consciously forecast the future and alter one's decisions based on it, as opposed to being guided by instincts. That is the ability only humans have developed so far.

I by no means believe that no other species can ever develop a similar ability. I am just saying that we do not know much about the conditions required for a species to develop it, so it is hard to predict what species, if any, will be the next one to do so on Earth.

There is no way to tell if humans or other animals would even develope past their already assigned intellect or that they would need to.  I think if people ceased to exist and the animal kingdom were to some how survive that they would be happy just being who they are and continue living their lives.

As far as who would replace us?  Who says we need to be replaced.

What in nature requires us?

“For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much—the wheel, New York, wars and so on—whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man—for precisely the same reasons.”