I'm ambivalent about this, so let me play devil's advocate for a bit.
On the one hand, yes, we absolutely need to partner with other sites to increase the visibility of gd.se. On the other hand, as I said elsewhere, we have managed to be at once too specialized and too generalized. The various narrow-topic sites listed as "probable overlap with gd.se" would, in several cases, be more productive than gd simply because they narrow the focus to more specific topics that, in many cases, tend to be complained about, but reluctantly accepted, here.
We grumble about "what's this font?" questions that would be at home, along with more advanced typographic topics, in a typography se. Potential partner sites like typophile.com, ilovetypography.com come to mind.
The Creative Suite tools (Photoshop first and foremost, but definitely including InDesign, Illustrator, Dreamweaver, et al.) are a major source of the questions that are asked here, but these "how to" questions have been debated as off-topic, on-topic, how-do-we-get-rid-of-them-topic numerous times. They are inevitably a major source of questions, if only because they are the tools by which most designers earn their living. One very popular BBS-type forum site, graphicdesignforum.com, has an explicit forum for Adobe Tools, which is heavily trafficked and often very useful. Numerous potential partners include InDesignSecrets.com, home of the excellent InDesign Secrets podcast and a personal favorite, Mordy Golding's excellent rwillustrator.blogspot.com, and Adobe themselves, who are always interested in increasing the intellectual commons that surround their products.
Questions about Open Source graphics tools show up here from time to time, and too often go without answers simply because most of us don't use and aren't familiar with them. A dedicated OS Graphics SE might attract a viable community.
Digital Publications is a rapidly-growing field, dominated by Kindle and Adobe's DPS, but rich in tools with new ones coming online regularly. It is a specialty that might easily become lost in the vastness of gd.se, but it is a fast-growing market. The only really useful sites at present are Liz Castros pigsgourdsandwikis.com and epubsecrets.com, an offshoot of InDesignSecrets.com devoted entirely to ePub and .mobi issues. This is a field where "under the hood" code issues are inevitable and essential, but would be rejected by gd.se as off-topic.
GD.SE comes alive (based on what I see in the voting) when questions about actual design or the creative process, as opposed to the tools of design, come up. They tend to get buried in a sea of "how to" software questions. Perhaps, with somewhere to send the tool-tip questions, we could allow gd.se to blossom as a site where design questions could be asked and answered. On that basis, we might pull the more serious-minded designers from very generalized sites like how.com.
It's hard to argue, as we are doing when we ask "we should broaden the scope of gd.se?", that questions about the most effective particle-generator plug-ins for After Effects and effective use of design grid systems for the web belong in the same forum.
For better or worse, we are in an age where fields of information and sources of information are increasingly granular. General news sites have had their traffic and relevance nibbled away by specialized competitors. "General-interest blog" is an oxymoron, and general-interest Q&A sites are populated only by the ignorant, ignored by anyone with an above-room-temperature IQ (Celsius). If you want to know about Curiosity, would you visit NYTimes.com, or nasa.gov/msl?
It seems to me that there is a debate we need to have about whether gd.se is simply too broad to have a recognizable identity in the broad community of design, and whether perhaps more narrowly-defined sites with narrower and more clearly definable topics would perform better. I see an analogy in the various specialized coding forums on SE, which could be argued as being already covered by SO but are clearly needed and viable.