Stack Exchange is growing rapidly, new Stack Exchange sites are emerging on a regular basis, and naturally there is more overlapping between sites on certain topics.
However, the build-in systems of closing reasons, marking duplicates, question migration, announcement of related questions seem to be more and more rigid with respect to this. Unless these systems are not re-scaled to improve the connectivity between SO sites, the community should be very cautious about off-topic closures.
To be more detailed about these problems and possible solutions:
When flagging a post as off-topic, I have the choice to suggest Stack Overflow and Super User. Only. Of course the "other reason" input field can be used to suggest something different, but why don't make it simpler? You probably know the variety of Stack Exchange sites - Please, let me choose from at least some more Stack Exchange sites, there are so many great ones out there:
For example, I have been involved in a question which dealt about data downloading. It is now closed and suggests Super User - but it could point to Open Data instead, which may be more related.
"Linked questions" and "related questions" should include related questions from other Stack Exchange Network sites. (This little column to the right.)
For example, there is a question about spatially normalizing crime data here at GIS.SE, and a very related question at Open Data. I posted the link to the other question as a comment - but it is not showing up in the "linked question" list. This is odd.
This is probably a bit more provocative: There could be special sister site tags (or an enhancement of certain, selected existing tags), which automatically link a question to an additional site of the Stack Exchange network. Have a look at Open Data again, lots of questions have tags like "geospatial", "GIS", "geocoding", "maps". We have the "data" tag in return. Similar scenarios can be probably found for other topics. (e.g. Stack Overflow has a variety of widely used GIS-tags as well.) Why not have a button like "overlapping from other sites" on our front page, which links to a queue with all those GIS-touched questions from other sites of the SE Network?
I claim this was more useful than the "badges" short-cut right beneath the headline, for example:
Stack Exchange has very powerful features (probably I am not aware of several features myself as I am quite new here) but they are not intuitive to find, especially combinations of them, which may multiply to amazing features. This increases the gap between power users and new or occasional users - who are likely to miss helpful features. Some of the ideas mentioned above can be accomplished manually by a versed user. The usage of favorite tags, for example, along with the possibility to filter questions of the complete SE network by tags basically results in a very custom front page. There are some extremely powerful search methods. Just to name a few things. I am missing an overview page that introduces all this great advanced functionality, which helps you to focus on the topics you are actually interested in, thus ultimately guiding you to the variety of SE sites and helping you to find the right SE site before you ask. (The SE 2-minute tours have a quite appealing design. Why not have additional, longer tours that explain more advanced stuff?)
I think the lack of these things is one of the main reasons why lots of off-topic-questions are actually being asked.
New users should be guided through their first questions in a better way. The "ask new question" page could help the questioner in a step-by-step way, instead of just having this "How to ask" box to its right. (Maybe limit this to the first 3-5 questions of a user)
This ranges from choosing a good title, what information to include in the question body ("How to frame a good question?" is a true classic), an interactive way to pick appropriate tags, but also encouraging the gardening of the question during later views, i.e. encourage to edit / accept / vote for the answers.
To focus on the specific question about on-topic vs. off-topic geospatial data again, the "Ask new question" page could present a list of the most relevant other Stack Exchange sites. Maybe this can be combined with the clarification of some on-topic / off-topic rules. (Information like this can be visualized creatively in a tag-cloud style to make it more appealing, less harsh and less text.)
With more and more Stack Exchange sites in the Network, the border between on-topic and off-topic subjects is quite unclear for a variety of questions. This is a discussion about geospatial data, but we could also discuss other topics here, which probably can be well answered by the GIS.SE community, but the question itself is not a pure GIS exercise. (e.g. certain software issues, like GIS software setup problems are posted on a regular basis here; also Python programming topics, questions about statistics - some questions would probably even be on-topic on multiple SE sites.) The off-topic criteria have to be realized in a consistent way throughout the site - if off-topic-ness is handled loosely on certain subjects and briskly on others, it is probably confusing to a [new] user.
"off-topic" is not equal to "go away" (Neither should this be interpreted by the questioner, nor implied by other users.) Migrating seems to happen quite rarely. Why? With the growing SE network, I would expect more and community-driven migration in order to "move oysters closer to the pearls".
Bearing in mind the wide variety of SE sites to pick for asking a (overlapping) question, the whole SE community could be more aware of interesting tags on other sites. (I will certainly visit Open Data regularly from now). The site design could encourage that. Even if topics of different SE sites are clearly bordered, the site communities should not encapsulate themselves.
With all these lengthy list in mind: I fear that GIS.SE will become more negative if the above issues are not addressed but off-topic rules are applied strictly. There is a recent stellar & famous recent at Meta.SO: Why is Stack Overflow so negative of late? - Both the community and the SE developers should try to prevent this.