I'm disappointed they never revisited these parasites, especially when they left it open with Datas line about the possibility of a homing beacon being transmitted. Even in a later series, there's potential for that to become a storyline.
And here I must again mount my hobby-horse and declare that Data was quite seriously mistaken about that signal.
No, it's not a homing beacon: these critters already very well know where Earth is. And their feared invasion fleet was in no way harmed by Picard and Riker's gunning down of the advance party; even if the main hive of the monsters is far away, it's up to that fleet to tell them where to find the big fat prey.
And no, it's not a signal saying "Come here". It's a signal saying "Remember that bit where we invited you? Forget about that. We totally blew it. In fact, turn tail and flee, right now, because these folks now know where
you are. And they shoot to kill.".
The invaders were already on their way in. They had this grand plan where manipulation of key decisionmakers would open an invasion corridor for a very brief while. They could hope to hold the bridgehead for the "quiet weekend" they spoke of, and then it would be all over, one way or another. Now it was over in that less desirable way, and the Remmick creature thus had to send out an "ABORT! ABORT! RUN FOR YOUR LIVES!" signal.
So the critters ran. What sort of a rematch could they ever attempt? They are one-trick parasites who bet their future on the victim not being paranoid enough. Starfleet now has paranoia aplenty: measures capable of dealing with Founders will make short work of adversaries like this. Worse still, the critters are dramatically uninteresting. So they seek peaceful coexistence? Aka possess and conquer? Trek has no shortage of the type...
Timo Saloniemi