What? You mean adding stuff - original or from other sources - to adventures?
If so, yes. All the time.
And when I DM, like in the Marvel Cinematic Universe, "It's all connected". So you'll get links/references/jokes to previous games I've run (wether you were playing them or not) or am running. And often hooks to future/other adventures. I'm also not at all adverse to campaigns going completely off the rails & into the wild blue yonder because of this.
For example:
Currently I'm running the
Reign of Winter Pathfinder campaign for my Sunday group. (their 4/6 finished now) Basic plot is that a very powerful Winter Witch is trying to usurp Baba Yagas power. The PCs get drawn into this plot when her ritual causes wintery portals to open all over the globe as she tries to bring about a new ice-age so as to channel elemental winter.
The PCs of course thwart this & then go on to hunt her down, etc etc etc.
But meanwhile, over in the 5e game I DM at the local shop on Thur evenings
(with completely different players)....
The 5e characters were on a desert/tomb raiding adventure involving lots of sand & mummies. En-route to the next tomb on their to-loot-list they awoke one morning inside a 5 mile radius of pure winter. 12 degree temps, snow drifts, ever worsening blizzard conditions, & strange creatures and arctic raiders who'd come through the portal the Winter Witch had opened in their area.
They had a pretty tough, but fun, time as they were not at all prepared for a winter adventure.
Near the end of the 2nd week of wintery deserty fun? The skies cleared & near 100 degree temps returned burning away all the snow & ice. The 5e party had zero to do with that, their characters had zero idea what the 9 hells happened, & only 2 of the payers (who knew I'm running a winter adventure for another group) realized that this was in reference to the Sunday game.
See, 4 days earlier IRL (Sunday), the PF party put a stop to the Ice-Age ritual.
Since then in the 5e game they've come across NPCs & other bits of info that've referenced the mysterious winter zones that came & went last year.
Now had the 5e party gone a different direction (further in vs escaping the area) while in the winter zone? They'd have found the actual winter portal and could've entered it.
And then they'd have been stuck in a kingdom locked in eternal winter, ruled by Ice Witches, & could have ended up racing the PF party for access/control of the fabled Dancing Hut. Since all my games are connected, that would've been
really interesting....
