Rice is not a prominent enough personage to be guaranteed a speaking slot at the convention. Her omission could be a signal that she's The One, but it's just a guess.
Like all the others, she will quickly be a lightning rod if named. She has a lot of 'splainin' to do, but ultimately nobody votes for Vice President, even if the Nominee has one foot in the grave, as Biden does.
And as with ALL the rest of them, if she were not a woman and not some "color" she would not be worth a second look for anything other than a sub-cabinet level position in the future.