Incidentally (digging deep into my memory cells here) Drupal has a synonyms module that you might look at for inspiration/ideas. As with many things Drupal it is too complex but the example screenshot is interesting - note how it makes it obvious at selection when the term you typed in is a synonym of another term. It might be jarring to folks to look for “pots” and come to a page of planters topics. Displaying the results as Planters (pots) might help address this.
The drupal module also has the ability, it appears, to determine whether a synonym can be selected when adding terms to content. That might be interesting. I often run into issues where people create tags that are similar to other tags - I wouldn’t mind if those showed up in results and sent people to the right tags page, but I am not sure I want people to be choosing to use the similar but wrong tag.
Another issue I run into frequently is with delimiting characters in tags, e.g. srilanka and sri-lanka and sri_lanka. All three are the same but do I want people to be able to add sri-lanka when the preferred tag is always srilanka? Because both are possible, unfortunately, we are constantly gardening and renaming tags.