Coleman was with Capaldi since his debut as the Time Lord, so her re-appearance would be a great send-off for him.
Coleman showed great respect for the show following her exit to the series, and when that’s compared with her respect for Steven Moffat, it wouldn’t be all that surprising to hear she cleared some things from her schedule to make a future cameo happen.
First, Jenna Coleman has frequently said that she would always be happy to return to Doctor Who if there was a good story, and a regeneration episode that features the First Doctor is about as good as it gets in terms of story.