>I once had, and never used, an idea that the consequence of using such technology was to accelerate dark energy, so that the more civilizations used FTL travel, the greater the energy requirements became. As if seafarers had to deal with the oceans expanding just a bit every time they were crossed.

I believe one of the aborted ending ideas (and dangling plot-threads) for the Mass Effect series was something like this.

Like use of "Element Zero" (the exotic matter unobtanium that powers everything, including FTL travel) releases some kind of "dark energy" that is bad for vaguely defined reasons; one of which being that it destabilizes stellar fusion and makes stars go nova prematurely.

So the "Reapers," or big-bads of the series, were an ancient civilization that set up a recurrent process of culling nascent advanced civilizations to prevent them from growing and using such technology to problematic levels.

They ultimately scrapped it and went with an "AI and Man can't coexist" angle instead.