Tagging might be the solution

https://meta.discourse.org/t/tagger-tags-for-topics/14573

Instead of messing up the category section, why not consider tagger plugin?

https://meta-discourse.global.ssl.fastly.net/uploads/default/4231/7e5e990c3ff80af1.png

I understand that there were several posts requesting for new category section and administrator was reluctant to implement them.

@SzAdmin, maybe this time you can consider the tagging feature in addition to category?