So, Merrin posted that on Instagram:
https://www.instagram.com/p/Bfb_YG8g995/?hl=fr&taken-by=realmerrindungey
"Ran into some Queens today... 🐙🐉🍎👑#ouat"
We could think it is a (rather big) coincidence that the three of them ran into each other... But if you look at the background of the pic, there is the front page of a OUAT script...
So, they met for something OUAT-related or somewhere OUAT-related. All three of them.
If you recall the interview that A&E gave when they announced the ending of the series, they did say that some familiar faces would return before the end of the series:
http://ew.com/tv/2018/02/06/once-upon-a-time-ending-season-7-series-finale/
So, this picture could be a coincidence and nothing... but it could also be a tease. A tease that Ursula and Maleficent are among the returning faces for the end of the series?
EDIT: And Joanna Metrass too! She met with A&E and Lana... At the same place where Merrin and Kristen were today! She posted videos and everything...
http://storybrookemirror.tumblr.com/post/171110153766/via-joana-metrasss-instagram-story
Are they doing the vignette episode?