One idea I've had about past civilisations is to do with quantum immortality. Suppose you did a Schroedinger's cat type experiment, but you were the cat. If the multiple universe hypothesis is true, then when the box is opened, two universes form, one in which you're alive and one in which you're dead. Since you're not in the universe where you're dead, subjectively, you would simply always survive the experiment. Now, imagine if you linked the trigger for death to a measurement of energy. You create a box that kills you unless some observed material randomly spikes in energy. It's a bit like a quantum suicide version of Maxwell's demon. Subjectively, you get free energy. More than that, this quantum killer Maxwell demon would be a fairy Godmother in disguise. You could make anything happen as long as it is directly observable and has non-zero probability. However, if you do it enough, anyone who isn't in the box with you, will very likely see you die.
So there's the problem. You have a source of free energy or entropy or free anything, but using it will make you appear dead to everyone you care about. The solution would be to get everyone you care about together in the same box.
From our point of view, when we explore the galaxy in the future and we will keep on finding long dead previously civilised planets which had advanced aliens who one day suddenly bundled everyone on onto a massive spaceship and then blew themselves up. Then one day, we'll start bundling ourselves up and do the same.
In practical terms "bundling everyone together" might mean merging into one consciousness (for convenience). Which would make the whole process a lot like "The Last Question" by Asimov.