Is the next step to change Subscription to Membership on the left-sideI think that "Subscription" there is a useful choice to make it clearly distinct, at least for moderators, from the Members page.
A distinction that I think may be semantically useful is to use "subscription" ("subscriber", etc.) in an email context, and "membership" ("member", etc.) in a web or comprehensive (both web and email) context.
Hence the Subscription page could stay "Subscription" because its controls are (almost) entirely about delivery of messages and notices to you by email.
The Members list, by contrast, is comprehensive: it controls members' activity on both web and email.
Other usages could be judged by that same criteria.