Again the reason for creating the “Development Issue Only” was exactly for this: to have a component for issues that are related to the development of XWiki but that shouldn’t be seen in the RN. Design issues fall exactly in this category, and I really don’t see what’s the problem of having a single category for this topic.
We should always do YAGNI until proven that we need something different. In short, do simple things (adding a new issue type is too complex since there’s no immediate need). I don’t see a single problem that would be caused by using the existing Development Issue Only component. Additionally if, for some reasons, we want to list only UI Designs issues (and I don’t think we have that need), we already have labels for issues related to UI: ui.
Last, issue types must not be about a domain. They’re supposed to be applicable to all domains, which would’t be the case. We had this discussion for Security BTW. If you want to quality an issue, labels are there for that.
Thx