We're not going to announce a release date until it's actually ready to be released, and I'm not going to guarantee these 5 articles - but that's how it appears at the moment.
With that in hand, however, we are starting to look at issue 27. We've had two solid submissions and several proposals, but could use more. We've had requests for a (simple) article assigning 4e domains to the GH gods; I'd also be interested in seeing something that updates the sub-races to 4e rules (ie hill dwarves, wild/valley/high or grey elves, etc). There may be other similar possibilities in Pathfinder, Castles & Crusades, or other rulesets.
OSR submissions would also be welcome - as close relatives of the original D&D rules, OSR articles are easier to adapt to a wide range of editions & rulesets than 4e or Pathfinder.
You don't have to be an expert on Greyhawk lore to submit something, either. The only thing better than reading about GH, in my opinion, is reading the fascinating things other people have done with their GH campaigns, and where they've diverged from "core" canon. If you are stuck on an issue of canon, the sage forum gnomes at Canonfire!, Dragonsfoot, and the Piazza can usually get you an answer in a few short hours.
Finally, not every article has to run 20 pages - if anything ,we need more short articles on everyday things like monsters, spells, magic items, artifacts, adventuring locales, NPCs, and the like.