Knowing spoilers from when they filmed this episode, here’s a theory I have:
After Rumple makes his deal with Hades, he decides that one of the best ways of convincing Emma to stop her mission is to introduce her to Milah. This meeting will likely result in Emma learning the truth regarding the complicated history involving Rumple, Milah, Hook, and Neal/Baelfire. Perhaps he hopes that Emma learning all of this will turn her off from Hook, but obviously it’s not going to work out that way.
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