Two factors keep revolving in my head.

1) Riddle1/Quirrellmort/BadVoldemort is basically the only "existential risk activist" in the story at this point. Handling the big risks responsibly so that his immortal self would have a world worth living in forever was apparently his deep motivation for taking over Magical Britain in the first place, and then it turned out to be easier than expected. Eliezer probably doesn't agree with Riddle1's tactics or other values, but it seems like this aspect of him has to come out well by the end of the story for it to do the moral and educational work that Eliezer probably intends.

2) Riddle1 probably thinks that the prophecy makes Riddle2/Harry/GoodVoldemort into the number one existential risk to try to mitigate, and he is probably wrong about this because Riddle1 doesn't know much about science or science fiction, which are my leading candidates for "the power he knows not".

HE IS HERE. THE ONE WHO WILL TEAR APART THE VERY STARS IN HEAVEN. HE IS HERE. HE IS THE END OF THE WORLD.

The stars aren't sacred. They are fuel and construction material. Tearing them apart (under controlled conditions) and using them for productive purposes is totally part of how the future will go if humans make it off of this planet and start acting like a proper post-scarcity civilization from science fiction.

Presumably "he who tears the stars" is Riddle2/Harry/GoodVoldemort, but whoever does it presumably has a reason.

Many chapters ago my leading theory was that Hermione was close to information theoretically dead (brain ischemia being a significant problem within relatively short time periods and her body had hours before Harry got to it), and her body could be brought back animated by a plausible reconstruction of her mind built from external third party evidence sources... but this could produce a sad simulacrum or a high quality person depending on details.

Hermione not having woken up yet leaves the "sad simulacrum" option in play still :-/

Under this model, Harry could have had a long term plan to do the reconstruction very well, by using star sized computers that use every atom on the earth as part of the evidence base. There are lots of other reasons for doing something along these lines, like all the other minds that it might be possible to reconstruct and re-instantiate by the same method, which would flow with the anti-deathist themes.

I'm not strongly committed to this precise theory, because magic appears to make conservation of energy violations possible, and might allow effectively infinite computations to occur without using the stars to power them.

But still there are magical conservation laws it appears, as with "Dark" sacrifice costs and potion making. Given that Harry might be able to partially transfigure spacetime itself via an insight based on Julian Barbour's "timeless physics", it seems like he might be in a position to sacrifice and manipulate all kinds of things in clever ways and thereby not have to literally use hydrogen for fuel like a savage muggle... but it might still end up doing something to the stars?

One latent possibility that occurred to me is that Riddle2/Harry/GoodVoldemort might end up being "killed" and have the horcrux system work a bit weird and so that Harry ends up on the voyager probe... which he might have more luck controlling than Voldemort did during his first period stuck there. I think Harry might end up dead dead if he was discorporated, because he was created by Horcrux V1.0, and the Horcrux V2.0 network might only save Riddle1 rather than just saving any and all Riddle copies... but it seems like there is play in what might happen based on the evidence we've seen?

If Harry ends up on the Voyager probe, it puts him quite a bit closer to "the stars". It gives him time to think and "spaceship priming" might suggest an incredible array of options... Like transfiguring non-critical pieces of the probe into anti-matter or nukes, and using them to power exotic spaceship drives.

This particular scenario seems low probability (because Harry needs his wand to do transfigurations still and probably won't have that on the probe) but it shows how Harry already has crazily powerful science oriented options if he aims at short term profit taking instead of playing along with his student role and trying to level up in all the areas of magic that powerful wizards are expected to work on through years of school in order to become well rounded.

Of course, there's the 37 dark wizards aiming wands at Harry at the cliffhanger ending. I'm not sure how that will work out, but probably Riddle1 has some plans :-)