Welcome to the Off-Season ... Now Get to Work
This shouldn't deter you from testing more subtle campaign variants with less-engaged subscribers, or from taking direction on cadence or frequency from highly engaged subscribers. By all means test as widely as possible, but keep in mind that their established patterns of behavior can shape the way you analyze the results.
Scrub Your List
And now for your inactive subscribers: Spring cleaning is in order, although efforts that start now probably won't affect your program until summer. The most important thing to remember about email list hygiene is that its goal is not to pare down your list, it's to maximize your ability to drive revenue from your email program. That means getting your messages delivered to the inboxes of as many potential customers as possible. Simply cutting off unresponsive subscribers won't do this.
Instead, create a win-back strategy to re-engage your inactives, and take more than one crack at delivering a message that resonates with them. Some email marketers are too quick to remove inactives from their lists if they don't get an immediate response from their initial message. We conducted research last year that found it took, on average, 57 days before inactives re-engaged after receiving a win-back message.
By starting your win-back process now you can provide yourself with enough time to verify whether inactives are a lost cause or just slow to respond. As for cutting your list, yes, before your most critical marketing season do remove subscribers that haven't interacted with your messages for a very long time and haven't been moved by a win-back effort. Their presence on your list sends a clear signal to mailbox providers that at least some portion of your messages are going to people who don't care about them, and this will absolutely influence your overall inbox placement.