So, Fusions (i.e. actual media-based fandoms in their own right, rather than non-fandom tropes like 'sharing a bed') that have become tropes: When does a fusion become an actual trope?
- The Sentinel (TV) - sentinels and guides. Many examples in SGA, like esteefee's Redemption series or her Finding the Path (H-50)
- Phillip Pullman's Golden Compass books - people have daemons. Daemonology (SGA) and Cleave (MCU) by trinityofone
- A Companion to Wolves by Elizabeth Bear & Sarah Monette - psychic bonding with wolf brothers and sisters. This has been written in several fandoms now as a true "always been this way" fusion: like dira's amazing Generation Kill Every Marine a Wolfbrother series, and vass's excellent fusion with Imperial Radch - Mouth of the Wolf', and the truly wonderful The tameness of a wolf bypetra - a Slings and Arrows fusion in which Darren, as ever, is utterly impossible.
- Ursula Le Guin's (A Fisherman from the Inland Sea) - sedoretu foursome relationships are definitely a trope, I think, but these fics are rare. A wonderful example is luzula's due South fic Puzzle Pieces. I wrote sedoretu-fic in SGA, but it was a crossover not a fusion, as I wanted team AR1 to react to the foursome culture as an alien experience, then gradually give in to it and form their own. An earlier famous sedoretu fusion is Imperfectcircle's An Ever-Fixed Mark (Merlin).
- Anne McCaffrey's Dragonriders of Pern books - the trope of psychic dragon-bonding - although most of these don't update the trope but set it in McCaffrey's medieval-style 'verse. Such as Winging It by harrycrewe (H-50), or the Weyrwolves of Pern series by rainproof (Teen Wolf). This Cam/John SGA one is an updated fusion though: With Wings Spread Wide by skieswideopen.
The proverb is "a rolling stone gathers no moss". Once upon a time staying in the same place was seen as a good thing, secure and steady, allowing one to 'gather moss' (wisdom? Nah, probably money). In the changing modern world, the proverb's now used to mean that movement and change (rolling on) are good things, that stop you from getting mossy, overgrown and stagnant.
And so to bed . . .