Although I think that the ability to point out new communities is useful, I'm not sure this is the right approach. There are a number of topics that cross multiple communities and communities (sites) are defined by their expertise.
There was an experiment a while back where the same (on-topic) question was posted across multiple SE sites (with coordination with those communities). I forget all the sites, but they included Software Engineering, Computer Science, Stack Overflow, and one or two others. The answers on each site were very, very different. To the point where depending on what you were looking for, answers on one site would be of limited value to you.
I think that suggesting communities at the time of asking would do a disservice to the asker, since they may not be in a good position on where to ask. If they have a good question, they should take a good first guess. People can always use comments to ask clarifying questions and point out related questions on the site (although hopefully they have searched at least the site being asked on first) or on other sites (on or off the network).
I'm torn on the idea of what to do when the community where it was originally asked may not be the best. On one hand, the answers may not be helpful to the asker. But if it's a good question, they may be helpful to someone else later. This is why I'm generally opposed to blanket rules about not cross-posting questions. One should not copy/paste questions blindly, but there are instances where it may not be a bad thing when done properly.
Some level of thought should be done to link related SE communities. I don't have any fully thought out ideas, but I do think that improving tag wikis (especially their discovery and searchability) would be one aspect of this. Maybe some kind of sidebar portion similar to "Related" that links to other sites instead of questions on the same site would also be a part of this. Putting a new page in the Help Center that links to sites (specifically their /help/on-topic page) that are similar (perhaps mod editable, somehow) would be helpful as something to link to if a question is on the border of being on-topic.
I'm just not sold on an in-your-face suggestion of other sites at the time of asking.