**Note:** *This answer now also exists as [separate feature request](http://meta.askubuntu.com/q/14982/367990) here on meta.askubuntu.com.  
Further updates and discussion should take place over there.  
If you like this answer, please make sure you also upvote the linked question. Thanks!*

# I have a completely different suggestion.

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* 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?
>
> ![example image](https://i.sstatic.net/8tcoK.png)
>
> 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.

---
> ## A: Stack Overflow
>
> 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.

---
... and so on, one answer for each possibly related site.