X-Men: Grand Design is a six issue series by Ed Piskor which will examine every decade of X-Men history. Piskor already has four issues of the six-parter down, and is working on the final two. The release date for issue #1 is now set for December of 2017. The announcement was made at the "Marvel: The Next Big Thing" panel at SDCC which is one of the many news-heavy Marvel panels at the show.
Grand Design is an all new announcement from the House of Ideas, and for once, this actually looks like it's going to be a "next big thing" as its more self-aware than we usually see of Marvel. Most new Marvel comics are often attempts to freshen up old characters with a hip new gimmick, but this time around, we're not getting anything "new" instead, we're examining the old. And that might just be the best way to make X-Men into something cool again. But then, what do I know? I'm a games writer. Rich Johnston has offered this, for you comics fans, "Reminds me of X-Men: Hidden Years."
We'll update you as we continue to hear more about this series, but it might be a while until we get more details as the release date is still a ways off, especially for a mini-series that is, reportedly, mostly finished.
Reasons for the delay are unclear, but there are a ton of possibilities. They might be saving this specifically to tie it in to something big going on in the mainstream X-comics, might be waiting on Piskor to finish the whole series, or they could have some arcane back-end issues holding it up.
There's also the drama. Always good to give speculators a long tail to speculate on, so that might also be part of the reason.