User-defined keyword for each thread

Invision Community can do it.

Please be tolerable about - when they don’t matter much: “side-by-side” or “side by side”.

I guess you mean topic? Discourse doesn’t have threads.

Discourse isn’t trying to be some other forum and often has different ways to solve the same problems. Are you using Discourse and find that you need the features you suggest?

Have you looked at tags? See How to create tag structures, tag category restrictions, tag groups and tag relationships, A comprehensive guide to Discourse tags, It’s Time We Talked About Tags.