1) You need to do something about inactive addresses on your list. How do you define "inactive" subscriptions? Have you looked for patterns in the data suggesting critical triggers that turn people into inactives? Can you use that insight to develop a "reactivation" program?
2) Have you thought about "actives"? Have you considered rewarding those who have stayed loyal and responsive over a long period of time?
3) When was the last time you actively solicited feedback from subscribers? Are you afraid of what they might tell you? Don't you see feedback as valuable intelligence and a conversation starter?