Here is my two cents:
I agree with Mojca that lots of accents (both uppercase and lowercase) would be nice to have, just in case.
I would really like to see the ascii versions of the keyboard symbols: circumflex, doublequote and tilde etc. (Texnansi normally has a sort of orphaned accent instead of the ascii symbol, which is not all that usable)
Then there are straightforward symbols like copyright, registered and trademark;
The currencies (at least euro, dollar, pound, yen, but preferably many more)
A set of itemization bullets and dashes (some fonts have square ones)
The 'text-style' calculus operations like multiply and divide, plus and minus, plusminus, arrowleft and arrow right, text fractions.
I guess there are still missing punctuation characters as well.
Also, as many units and symbols that can appear in running text as possible please. centigrade and perthousan, but also some greek letters like micro, Ohm, alpha & beta (biology), gamma (radiology). These symbols are normally considered part of the text, so they should not depend on math mode. For example, if your write \beta-blocker, you want the \beta to be bold sans-serif inside a bold sans-serif section head.