The problem - if indeed it IS a problem - is that there isn't much a template for this type of female artist to follow.  
First, let me make a distinction between a singer (like, say, Janet Jackson) and a musician (like, say, Prince).  I'm not putting one over the other, but I do think that they won't have the same options.  
Musicians are usually able to age semi-gracefully.  They can perform an acoustic set, for instance, or perform more "adult-oriented material".  (In this case, that means aimed at the over 30 set, not sexually explicit material.)  These sorts of things tend not to sell in the same range as they did in their prime, and they tend not to win them any new fans, although they oft make their already-established fans happy.
These options, or something similar, ARE open to singers.  Singers can mount Vegas (or Vegas-style) shows, for instance, or they can do an album of "older songs' or standards.  This is the sort of path we've seen Celine Dion take, and Cyndi Lauper has tried the "old song" path with various degrees of success.  
But the problem with the female vocalists in the here and now - Madonna, Janet, Mariah - is that many of them have made their bread-and-butter in the club/dance scene.  They've all at least attempted something that might be considered "mature".  Madonna's "Oh Father" and I'm Breathless nostalgia exercises, Janet's "Again", and Mariah's Charmbracelet all might be looked as as attempt to push them outside the dance arena.  But for whatever reasons - midlife crisis, faltering record sales, what have you - they've all been sucked back in.  Madonna and Mariah's last album were attempts to stay current (and not entirely successful ones, IMHO), and both have been relying pretty heavy on their producers and ProTools for the last several years.  Janet is a bit more organic, but she's been stuck in that "I'm a sexy woman inside" schtick for so long, it sounds like she doesn't even know of anything else to do.
And to be honest, what CAN they do?  What would a GOOD dance-artist-in-her-40s album sound like? I think Whitney's latest, while not perfect by any means, does give some indication.  Bring it down a notch.  Accept your current vocal range.  Don't pretend you're something you're not.  Not everything has to be a ballad, but that doesn't mean the uptempo songs need to be club tracks, either.  You CAN write and record an uptempo song that isn't club-ready.  
 
Lex