You know this is my own shot in the dark and I know it'll never happen.....but i do enjoy sparking a debate.  What about....The Damned??

the mystery chapter that always seems to show up when needed to help turn the tide and save the day

I'm somewhat surprised the Crimson Fists aren't in there, considering that fluff wise, they're supposed to be the most numerous chapter to give members to the Deathwatch.

Okay 5 for 5 as far as the space marine codexes are covered...

Ultramarines, Space Wolves, Dark Angels, Blood Angels, Black Templars...and chapter no.6

Perhaps once the designers have posted the identity of the sixth chapter can then focus on other aspects of DW to tease the punters/consumers with. I would like to perhaps see a map of the Jericho Reach, more revlatory discussions regarding new aspects of DW such as the concept of Renown or perhaps more insight into the actions of the DW within Jericho Reach, perhaps some personalities in the great crusade within the Jericho Reach, among other posssible aspects before the introductory adventure rolls out prior to the release of the book...

Interesting facets of the setting and the mechanics, which if are on par with DH/RT then should indicate that DW will be of very much the same standards as its predecessor.

Previously I would like to see the Iron Hands inclusion, or possibly the Hammers of Retribution from PtU (interesting to read the accompanying fluff on the chapter), but at this point I would just like it to be done and dusted because the process has dragged on a little long. At present any of the other chapters which don't have a codex for WH40K (bar the Legion of the Damned) are equally possible for inclusion...

Coming in no.6, your time is up!