I was definitely pleased to see the suggested values feature rolled out. I think it might be helpful to also have a type for parameters that should be one of several values but nothing else. Sorta like boolean only for more than two.
Topic on Extension talk:TemplateData
We are happy to hear you like it. A more strict version is still discussed in phab:T53375. For now the team decided to go with the most flexible version for a multitude of reasons. It already solves the vast majority of use-cases, without ever blocking anybody. You can find more about the reasoning in phab:T271825 and it's sub-tickets.