I think we need to look back at things that were discovered previously to look at what rumours might be used for.
If we had the codex at the time, how would these have been presented
After Jaques failed jump to Beagle point, his distress transmissions could have given clues that appeared in the rumours section, giving clues what to look for, perhaps[s in several sector candidates so it wasn't too much a clue
After their "Bribery" CG this chap could have given the clues we got as a rumour in the codex to find the crashed Thargoid ships
Rumours rather than being lucky enough to meet them in game
Halsey's visions were supposed to give us clues to find the Guardian worlds so would have been perfect
Barnacles and UAs would definitely been first appearing as rumours, and natural curiosity around Merope would have then lead to the rest.
The Dynasty projects and Rift story as rumours
All the same clues we got be presented in the codex, supplemented by Galnet News stories
So it isn't just rumours to find things unfound in general, but rumours to find things that are connected to the story arcs
I am still hoping we will get a Galnet news article about a transmission from Gan Romero which becomes a rumour leading to the next guardian discovery and the Sisterships to the
Saravati
But I think we need to wait until there is a next step in the story line that needs the player base to find something we haven't, which may not be the case at the moment