Jay-Z - We know he's been involved in the making of the new album, and he appears on 'Suit & Tie', so Jay-Z ia a forerunner to be one of JT's special guests.
Beyonce - Like her husband, Beyonce is rumoured to have been involved in the making of The 20/20 Experience, and has confirmed the two have been in a studio together. Whether she'll have time to put in her appearance while also on her own world tour is another question.
Timbaland - Another regular JT collaborator, Timbaland is thought to have been heavily involved in Timberlake's new project, so if he popped up on the tour it wouldn't be a massive surprise.
The Neptunes - It's been a while, but The Neptunes produced JT's first, R&B flavoured album alongside Timbaland. With the popstar apparently returning to R&B slightly with his new material, Pharrell might just be one of his special guests.
Christina Aguilera - Perhaps a bit of a long shot, but Christina and Justin have shared a headline tour before with the Justified/Stripped Tour in 2003. Aguilera is trying to kickstart her career at the moment, though with markedly less success than JT. Perhaps an appearance during his tour is just what she needs?
Snoop Dogg/Lion - Back 2005 Timberlake recorded 'Signs' with Snoop. This year Snoop's due to release his reggae album Reincarnated. With many people already sceptical, perhaps he'll try to raise his profile with an appearance on The 20/20 Experience Tour, although that's not likely to help with reggae fans...
Nelly Furtado - Justin and Nelly both provided vocals for Timbaland's 'Give It To Me' back in 2007. Perhaps Timberlake will be recruiting Furtado to perform alongside him again during his tour.
T.I. - T.I. and Timberlake have worked together several times, both on JT records and T.I. records. For the rapper to appear during the tour, and indeed on the album, wouldn't be a shock.
Will.i.am - Will was a contributing prodcuer on 2006's FutureSex/LoveSounds album. He's recently been making it his business to work with as many pop stars as possible, like his recent collaboration with Britney. Perhaps JT is next on his list.
Elton John - Elton's 31st album is due for release in March, as is Timberlake's third album. Justin played a young Elton in a music video once, and if their respective tours happen to cross paths, just maybe Elton will put in an appearance with JT.