Ok this topic covers a large range of confirmed and speculated tie ins that will be introduced in Season 6.
Previously confirmed ties between the 2 shows (Prior 2 season 6)
1. Appearance of Leroy and Ashley, and Emma's bug driving (which would imply Emma, even if JMo was not on the set) in "Down the Rabbit Hole".
2. Maleficent, Robin Hood, and the Merry men, appearing in "Forget Me Knot"
3. Ruby and Cora make appearances in "Heart of the Matter"
4. The Catterpiller appeared in both shows.
5. Will Scarlet joins the main cast of Once in Season 4.
Confirmed ties in Season 6
1. Jafar appearing (at least in backstory so far), probably a main guest star role for at least the first several episodes.
2. Dr Lydgate appearing in "Strange Case". (Probably backstory only, but who knows)
Now, for the fun. What other potential tie ins do you see occurring during this season?
1. Final resolution of the relationship between Ana and Ella (implied but never confirmed).
2. Final resolution of what happened with Will and Ana?
3. What happened to Jafar AFTER he became a genie?
4. What else?