Extended fonts don't get used very much
It seems like there are a great many condensed fonts out there, and not many extended ones. I was thinking about this yesterday, and realized that I very rarely use extended fonts in my work. I can think of a lot of reasons why this might be the case, but still, could there be an opportunity here?
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They take up too much darn space! ;-)
Seriously, though. I've almost never worked with a client who did not want to put as much information as possible into the smallest given space. You just don't have this economy of means with "extended" weights.
However, in display typography, there are quite a few wide typefaces that don't belong to superfamilies. They are extended, but don't bear the "extended" moniker in their name. Maybe this helps keep them in parlace?