Featuring G5 background character Bucket (placeholder name voted on through YouTube Poll) aka. Shiny Sparks (alternative placeholder name voted on through Derpibooru) inspired by their re-occurring alicorn animation errors (now with a G4 kirin form).
Also featuring an older version of the G4 background character Alula aka. Princess Erroria, inspired by her slowed aging, famous alicorn animation error and subsequent alicorn Easter egg.
Its confirmed in the episode ‘Fame and Misfortune’ that the story of when Rainbow met Daring Doo is one of her favourite stories from the Friendship Journal:
“Princess Erroria”: Aw, we don’t wanna hear her bowing lessons! Come on! Tell us again about when you met Dawing Do!
Rainbow Dash: [laughs nervously] Again? Haven’t we already covered that one? A couple dozen times?
As such, it made sense to me that the grown-up Alula might eventually track down the ruins of the destroyed Fortress of Talacon where Rainbow Dash and Daring Doo first teamed up.