If you have a successful mature community, there are broadly 3 channels of growth.
1) Greater level of penetration within the existing audience. You try to further expand the community's penetration within it's existing audience. This has limits. Sooner or later you reach the saturation point. Everyone that is likely to be a member will be a member. You hit a law of diminishing returns of targeting that audience.
2) Expanding into related field. You move into a sector that's related to yours. Facebook began in the ivy league, expanded to all colleges, expanded to all students worldwide, then became an open network for everybody. Mumsnet launched Gransnet. There are plenty of examples here.
3) Expanding into new cultures/languages. You target countries where the language barrier is prohibitive. This is the big, risky, bet. I can't think of a single mature community that has achieved this yet. The big problem is we think members prefer to speak in their own, native, tongue. That is true.
But given the choice of participating in a mature, established, community in their second language or participating in a new, fledgling, community in their first…they pick the mature community every time. For a move into a new language to succeed, it's about finding groups that have a high level of interest but don't also speak English. That, for example, makes most of Asia a better target than most of Europe.
Too often, we focus on just the first. The latter two are riskier but have far greater potential.
p.s. Just a few weeks left to sign up to our community management conference in London on Feb 24 – 25. We hope to see you there.