When working on my current project, Science Gothic, early on I played with having the weight, width and contrast axes for the notdef. It looked cool! But then I realized that my notdef would not stand out as much, and that the whole point of the notdef was to look out of place and call attention to itself. That way one sees it when proofing or examining a document.
So I reverted to a generic notdef.
Then recently one of my colleagues working on the project, not knowing this history from early on, did the same thing again, and made a variable notdef that has its weight and width axes. He is less persuaded by the argument against it.
I am curious what others think. Should the notdef be highly generic so it stands out, regardless of font style? Or should it be font-specific and blend in?
I usually match the height of my .notdef glyph to the Latin cap height, to ensure that it has good visual impact within lines of text, but other than that it shouldn't reflect the design.
And: at least Adobe, Microsoft and Apple all use notdefs that do not vary to match the weight/width/whatever of the host font.
In any case, I don't see a scenario in which the .notdef glyph looking a particular way is helpful. Either the user already knows what the font is, or he or she is presented with a .notdef glyph that, unless he or she happens to be a connoisseur of .notdef glyphs and can recognise a particular version of Helvetica from its .notdef glyph, might as well be generic.
Now, that said, when I say 'generic', I don't necessarily mean a uniform glyph in all fonts from all vendors. I do think it's appropriate for individual font makers to have their own .notdef glyph design within the conventions of the empty box (tofu), box with an X, or box with a ? (my preferred form).
This is the current iteration of my own .notdef glyph, which I space so it created a block of boxes if a sequence of characters is unsupported.
And I really don't think putting your foundry logo in the .notdef glyph slot is a good idea. Apart from confusing users as to what it means, do you really want to be trumpeting 'Foundry X couldn't be arsed to support this character'?
Greg tells me that he asked Mike, Ian, Sue, and Geraldine each draw one, with the only constraint being that it had to be the same size or smaller (data size in file) than the largest glyph in the font. His recollection is that the above were drawn by Mike, Sue, Geraldine, and Ian, respectively.