Some ideas:
* How do people organize their email, if at all?
* Do people practice "Inbox Zero" ? How do people manage their email inboxes?
* Do people subscribe to email lists and newsletters, and how are those managed?
* Do people use Gmail's "smart features" - smart reply, for example?
* Do people use filters? If so, how so?
* Do people use the Gmail snooze feature? If so, how so?
* Why (or why not) use Gmail instead of the email from your ISP, another free email provider, or a custom email address?
That's off the top of my head :)
Peggy