Dunmurderin'

Humans discover ancient and extremely enigmatic alien relics around the Solar System. On inventing plot-enabling As Fast As Light starships (PEAFAL), humans determine pretty much any system old enough has relics from the Whoever They Were (WTW). The WTW showeed up in the early Proterozoic, did their thing for 300 million years--although not on Earth, as far as anyoe can tell--and then vanished seemingly overnight for reasons that at as yet unclear.

They seem to have been interested in smaller terrestrial worlds, many of which now have life forms whose last common ancestor was six billion years ago. So probably they were xenoforming worlds? But apparently only barren worlds, for some reason. Also, if they used the PEAFAL drive, there's absolutely no evidence of it.

Age is one reason why the WTW are very enigmatic. 2.5 billion years of radiation and micrometeorites has turned all their artificial stuff into scrap. Sometimes, into subtle chemical traces in regolith. Nobody has ever reverse-engineered WTW relics into something novel to us. In fact, nobody is sure what the WTW even looked like (there are a couple of candidate remains of things that might have had big brain analogues). So, they make a nice Rorschach test for scientists to project their issues onto.

Added later:

Opinions on the WTW vary from "they were nigh-gods" to "they weren't actually intelligent at all" to "they are a Satanic plot."

PEAFAL ships interact with the interstellar medium (ISM) in ways that piss off astronomers specializing in the ISM. PEAFAL wakes could be detected at galactic distance but no non-human wakes are visible. The deal with the ISM means the longer the journey, the more likely it terminates in an energetic event somewhere in deep space. Effectively, this means there's a 1% chance per light year traversed of an unplanned terminal energetic event, which can be reduced somewhat by sending ships in pairs: one (presumably automated) trail blazer and one survivor. This is just annoying for robot probes but is an inhibiting factor for crewed starship recruitment.

PEAFAL ships are sufficiently expensive nobody builds huge ones. As well, nobody knows how to make closed cycle life support systems (LSS): the longest anyone has gone before an isolated ISS fell over and died is 20 years. Efforts to establish colonies on other planets have been very educational.

A mother's determination allows her son to finally meet his long-lost father.

Tripoint (Company Wars, volume 6) by C J Cherryh

Space wizard cultists but instead of one sanctioned cult and one forbidden cult, there are hundreds of space wizard cults, each of whom is convinced they have the best space wizardry. So they're continually fighting to see whose is better.

The Space Emperor's antipathy is due to the disruption caused by incessant space wizard cultist fights.

This all-new Critical Kit Solos Bundle presents Be Like a Cat, Be Like a Crow, and other one- and two-player tabletop roleplaying games from designer Tim Roberts at UK games publisher Critical Kit Ltd.

Bundle of Holding: Critical Kit Solos