I disagree: making new tags should be acceptable, normal, and not discouraged. Individual thought is trumped here by community hive-mind, but the hive-mind must be fed raw material to do anything useful! We want individual input, even imperfect input, else we starve the SE engine.
The community will fix improper tags as soon as the work is warranted. Anything that discourages the organic growth of the tag folksonomy is something I'd oppose. We want people to make tags that they are experts in, even if they're not experts at using RPG.SE yet.
Besides, even if I agreed with the principle, I'd disagree with the proposed feature. Overloading a UI convention like wavy red underlines to mean anything other than "there is a spelling error here" is bad design. It would convey "you spelled this wrong", not "that tag doesn't exist". The meanings may be arguably close, but the closer a distinct meaning is the more the UI semantics need to be different to avoid user confusion.