I implemented a related content feature for a client, but it went in a very different direction to this issue.

First, I identified some problems (and solutions) with just naively using keywords (tags)...

Problem: Hard to consistently use the right keywords.
Solution Allow auto-complete when typing keywords.

Problem: Hard to see what a particular keyword is shared with, when adding that keyword
Solution: Include an overlay to show everything current with that tag, and also provide a link to do that search

Problem: Don't want automatic association with minor keywords that are very common on one particular website.
Solution: Checkbox to define whether a particular keyword is used for related-content

Problem: Don't want association with old content with precedence over new content, just because it matches a keyword.
Solution: Bias towards newest content, with a limit on how many related content items to show.

Problem: Don't want stuff from completely different categories coming through just because it matches a keyword (categories may hold very different content).
Solution: Scope related content to particular groups of categories (i.e. particular news categories and forums)

Problem: Inputting keywords is tedious and feels like a step into the dark.
Solution: Auto-generate keywords before the article is even submitted (currently it happens after submitting). Have a link to click to show recent keywords, popular keywords, etc, so there's something to easily reference against.

This would all have been a lot of work, and led to a system that may be more complex than its worth. Defining relationships manually may be better, which is what I did.
I've attached a patch to this issue that achieves that. It was written for ocPortal v9. The module overrides in there are just to get the relationship field into the UI, as well as saving functionality for that.

Alternate patch for v10 attached (previous was for v9).
This has fewer content types, but also Comcode pages which the previous didn't.
I left out any changes to the CMS modules as they are similar.
There are also some relevant tweaks in here.

Not very helpful for me to be posting multiple patches that aren't really useful to anybody, but they can both be reviewed in the future when we try and add something proper to Composr.

We should use ARIA role="complementary" on the HTML.