They didn’t exactly add the mule arc early, though, given that his existence is only revealed by precognition.

I think it’s nice that the Mule isn’t so unique and unexpected in the TV series, actually. Seldon didn’t account for this kind of “psychic” capability in his equations, but now that he’s actually doing experimental psychohistory stuff like this is starting to pop out of the woodwork and reveal itself through glitches in the original plan. I suspect this is a benefit of working from a completed series rather than making up the series one book at a time like Asimov did, you can know ahead of time what to foreshadow.