That'll be coming in the Disney XD animated reboot of the original trilogy!! A female teenage Vader
But seriously, whoever eventually replaces Jones as Vader's voice won't really matter much as any one's voice can be digitally altered to sound near identical. There's been video games and other things here and there with Vader speaking in someones else's voice and it's hard pressed to tell it's not James Earl Jones. But hopefully he stays around for a while and Disney has the forsight to have him record tons of dialogue for future recordings that can be digitally mixed together to have him say anything they could ever want... They did that for Robin Williams for the genie in Aladdin. They wanted to make a 4th one and have enough dialogue from his recordings that they don't actually need him, but they can't because of his will decreeing his voice and likeness can't be used.