How about sketch some narrative structure of this bit of plot? Instead of starting with mechanics, start with the flight path of the roc. Because the PCs aren't going to be controlling the roc, this is a natural airborn railroad.
We can introduce an interesting story. Then we can attach mechanics to it in a way that makes the story better, instead of starting with a mechanical goal and attaching a story to it.
At each node in the story, we can attach
interesting consequences to the mechanical aspects as a bonus.
"Not fall off" becomes a narrative aspect instead of a mechanical one. While the story is about crossing the mountains without falling off, the mechanics
don't have to be about not falling off. You are already in a dramatic situation (on the back of a Roc).
So what interesting things can happen as you rode a Roc bareback? What interesting places?
Brainstorming:
- Wind
- Storm
- Lightning
- Sleet/Ice
- Hail
- Rain
- Cold
- Sunlight
- Mountains
- Cliffs
- Waterfalls
- Rivers
- Trees
- Valleys
- Other flying creatures
- Other not-flying creatures
- Other creatures on the roc
- Roc riding another creature
- Mountains/terrain as a character
"Lasting" Consequences:
- Damage
- Exhaustion
- Combat
- Social/political
- Information
- Consumable resources
- Death
The Roc could visit various "factions" of the mountains (need not be creatures; a storm spirit is a faction). How the PCs have to deal with each faction could determine longer term plot stuff.
At least one use of combat mechanics from the back of the Roc. These can even be length limited; the Roc flies down a valley, and Giants throw boulders at the PCs. PCs don't have to kill or defeat Giants, as the Roc will leave the valley in a few rounds.
Checks to mount the Roc securely and defend against weather, where failure results in Exhaustion levels.
The spirit/Roc can even give the PCs explicit challenges. Like, "This storm is bored. Entertain it or it will make you dance with lighting."
Some creature wants a pine cone. Get one as the Roc flies over the trees.
At least one thing goes "wrong" (unplanned). PCs have to do something to save the Roc.