I think since Christina is apparently pregnant, they will try and get Shakira and Usher to do another series back to back, if their schedules allow.
I really don't want to see will.i.am. I'm not a big fan of him on the show after 2.5 seasons of The Voice UK. He is also doing The Voice Australia series 3 this spring replacing the departing Seal, so I'm not sure he could squeeze the US version in before the Australian version finishes in July. I've enjoyed Kylie Minogue on Voice UK this series and she is replacing Delta Goodrem on Voice Australia this spring.
Joel Madden is great on the Australian version and is probably better known in Australia than he is here now, plus he is also doing The Voice Australia Kids version after the regular version finishes this year.
I think Pharrell would be pretty good, maybe Kelly Clarkson