keep in mind that of all aspects of a performance, the public historically seems to grant most flexibility to the artists vocal delivery. Consider artists like Bob Dylan, Tom Petty, Sonny Bono, Kenny Rogers and many others whose voices are anything but pristine... yet, they are distinctive... and for an artist it can be advantageous to cultivate a distinctive sound even at the expense of other things.

Having said that, the TC Helicon voiceworks PLUS (not the regular voiceworks) has a built-in formant modification feature... which lets vocalists "sculpt" the aspects of vocal quality that would normally be solely based on the physical shape and size of your vocal chords, nasal cavity etc.

But it's pricey. Cool though. I know of no other hardware unit that offers the ability to sculpt formant on the fly.

(For the studio, Melodyne has a formant modification mode.)