legitimate interest is, for example - have some way to identify user who is logged in. So keep email address for logged in users. Have some way to identify people who are trying to get account that have been banned, so have a table of banned users with email addresses for example.

none of these others are legitimate interest. Furthermore combining the data from legitimate interest (email address to keep track of your logged in user) with illegitimate goals such as tracking across services would be illegitimate.