Publishing is evolving and, of the big publishers (The Lancet, Cell, etc.), no one is more forward-thinking than Elsevier.   

They recently announced Article-Based Publishing, their new way to  publish articles as final (and citable) without needing to wait for the full journal to be complete.  Article-Based Publishing is the assigning of final citation data on an article-by-article basis, separate from production of the journal issue.

This is a big step forward.   The traditional model, for both pay-to-subscribe and pay-to-publish journals, is that articles are published on a recurring basis when the issue is complete.    Obviously it has worked well but with a gradual change to online publishing it's time to think in terms of something new.

Article-Based Publishing, they say, will still be articles in final form with volume, issue and page numbers, but not having to wait will speed up publication by an average of 7 weeks because the entire issue need not be finished.

They also say they intend to make it possible to have multiple issues in progress at the same time for some journals starting next year, making theme issues easier in the course of a regular schedule.