The legate has told me to go to Riften, and my journal says that the next thing to do is to "Regain the Rift", but there's no quest arrow telling me to go to Riften and no one in Riften that I talk to will further the quest. Can anyone help me here? I posted a few days ago with the same problem and received no replies. Thanks.

Check your other quests. There is a new one added that you'll have to complete first related to the Empire and Riften.