I suspect that I am going to find myself bouncing back and forth between the Cepheus Engine thread and this one.
For those who have not heard of the Krell, you might want to look at this thread, and then check Wikipedia and YouTube.
https://www.travellerrpg.com/index....orbidden-planet-as-a-traveller-setting.41955/
I have the Krell as predecessor space travelers in my sector as a way of putting Earth creatures out there, and they are clearly about off the chart for Traveller. As the planet Altair IV is portrayed as arid, the wide range of climates and biomes of Earth fascinated them, so they collected flora and fauna, but no humans, to stock various planets in the sector. I still need to figure out where they originated at, as the planet is now an asteroid belt. However, Krell artifacts are still being found.
Another groups of prior spacers are the Bald Spacers from Andre Norton's Time Trader series, three of which are on Project Gutenberg, while Galactic Derelict can be found elsewhere. They did collect humans, along with other creatures and transport them to other planets as low-leve l workers and zoo curiosities. Therefore, odd colonies of humans can be found in the sector, with no idea as to how they got there. Ruins of the Baldies also exist. This is why one of the "wild talent" Psionic abilities is "age sensing", allowing one to gauge the relative age of two objects, and with greater ability to gauge the actual age of the object.
Then you have the "vatches" from James Schmitz'
Witches of Karres, existing somewhere in Jump Space/Hyperspace, playing more of the role of gremlins rather than actually trying to kill players. To deal with them, you should read the novel and decide if you want to add them to the mix.
Then you have the planet, Triangle. Think Bermuda Triangle. All sorts of odd people have arrived from there, ranging over centuries of time periods. Who knows, you might find a flight of WW2 Avenger torpedo planes awaiting you.
You also have the planet, Wyrd, based on Andre Norton planet Limbo, in the book,
Sargasso of Space, another very good read. There is Sargol, read about it in
Plague Ship, and Topaz, from
Defiant Agents. You might get the idea that I like Andre Norton. You would be correct. Also H. Beam Piper and James Schmitz. All those might give you some idea at to where my mind, such that there is, runs.