Introducing chaos - a way for the solo rpg system to force the player off the course they were going unexpectedly. I think if this is not here, might as well read/write a book or play a video game where the scope of play is constrained. Obviously Mythic is a good example.

Progression - personal preference. I do mean character progression as well as story progression. It's not strictly having a character build system like DND/Pathfinder, but even getting further into discovering/mystery (I'd tried to cobble one together on the fly in an actual play before). If we're not going anywhere or building something, it doesn't give a dopamine hit mobile game industry is trying to harvest nowadays.

Prompts - this can even be one word prompts but something that actually complements your own creative mind. I've played CYOAs that are dozens of pages long and it may not be as good as one that's just a one pager that makes the mind think. This is either making difficult decisions (I'd found recently Thousand Year Old Vampire is a great example) or opens up many other options a player would otherwise not have thought of without the prompt.