The moderation team, along with the community here, can help with tagging, where necessary. I believe it’s also possible to put some quick links at the top of a category so users can quickly filter by tag.
What I do know is that adding categories in any forum is something that has to be considered heavily. Categories can be difficult to change or re-organize when there are changes (like having new popular game engines or old ones falling out of favor). Also, it’s not a pleasant experience to visit a forum and be overwhelmed with a wall of categories. And, as seen in a few posts already, the semi-permanence of categories raises the question of which specific engines get their own category and which ones don’t.
In contrast, tags are more flexible and extensible in a dynamic environment like this. Quick links to tag-based filters can be changed easily. Nearly any game engine can be supported/added. The general category list remains neat and tidy. Those are the main reasons I’d prefer to see just how much we can do to make tags a pleasant experience.