I have answered Add Elementary OS Stack Exchange to the off topic banner with a proposal which got a positive feedback, so I'm now writing it as separate feature request here:
Make a more flexible and user-friendly OFF-TOPIC close banner
What about rewording the banner to something like
This is not about Ubuntu.
Please visit our [off-topic compass] to find out on which partner site in the Stack Exchange network your problem might fit best.
And then create a nice community-wiki site here on meta.askubuntu.com, where the link in the banner points to, which looks like this:
Q: My question was marked as off-topic, where should I ask it instead?
You posted a question here and excitedly waited for an answer, but received a close banner like this?
Read through the answers below to find out where your question would fit best!
A: Unix & Linux
Every question about any UNIX or Linux based system is allowed here.
However, if you use for example Ubuntu, elementaryOS, OSX or Android, there are specialized sites for them, where you can get more specific answers.
All general programming questions should be asked here.
If you have working code and just look for optimizations, you're better of at Code Review though.
A: Super User
This is the most general site all about computers and related stuff. Every operating system is welcome, and so are hardware problems.
If you're looking for anything other than Windows or hardware stuff, you can probably find a more specialized site for your question though!
... and so on, one answer for each possibly related site.
The main reason for this is because we have so few space in the banner, but so many related sites in the network (Super User, Unix & Linux, Server Fault, Stack Overflow, Programming.SE, Code Review, Ask Different, elementaryOS.SE, Android Enthusiasts, Electronics, ...
We should help our new users to find the best place for their questions, as it's not always as obvious as we experienced users might think. I often still have problems with the scope of other sites as well...
What do you think about it? Feedback in comments, answers and votes appreciated!