I think that to the best of anyone's ability to tell, the speed of light is ultimately fixed and represents the speed of causality. I think that we can do a lot within those limits, much of it beyond belief today. That said, I truly doubt that we're going to exceed the speed of causality, create time machines, and that kind of thing.
For the artificial black holes, Dyson spheres, antimatter, quantum computers, and giant accelerators to actually work, the laws of physics they're based on can't be that wrong. They can be incomplete, requiring their place within a greater theory, but not simply wrong. There is observation consistent with the notion that the speed of light has been constant for the last ~14 billion years, everywhere we can see.
Imagine that human civilization survives and progresses for 100 million years. Do you think that they'll be able to look back on any of our "laws" without laughing?
At that point we'd likely be far beyond Dyson spheres, harvesting large quantities of antimatter, solar system scale particle accelerators, artificial black holes, quantum supercomputers, etc., etc. Whatever would be discovered is almost guaranteed to be unimaginable by us.