We know MJ liked to evolve his sound slightly with each album. So, since he already did R&B, that's out.
Assuming he waited another 5 years, it would be 2006. Biggest songs that year were Gnarls Barkley's "Crazy" (UK), Daniel Powter's "Bad Day" (US), and stuff like Nelly Furtado, James Blunt, Shakira and Scissor Sisters. So basically, a split between dance music and acoustic guitar.
For me personally, I wouldn't have wanted hiphop, or an album that was mindlessly packed with collaborations. That ages really badly. Besides, MJ didn't have the street cred to pull that off.
He would have been nearly 50 - maybe a classical album wouldn't have been a bad idea. And I think he would have recorded the whole album in 5.1 surround sound.