He should stay with Def Jam....
Why go to any other major label? Atlantic sucks, Universal is the home of Wayne, Birdman, Drake and the "whatever's hot at the moment" movement and didn't promote Distant Relatives AT ALL. He's already been on Sony (and as stated on HHID he's not going back) With Def Jam he's three number 1 albums deep, 2 have gone gold and LIG will follow, several Grammy nominations, and an Emmy award win. Def Jam pretty much has allowed Nas to be the artist he wants to be (aside from the Lost Tapes 2 fiasco

) why not stick with them?
Going independent for an artist of Nas's stature isn't really necessary. His last deal netted him him $10 million, he tours (co-owns Guerrilla Union which does Rock The Bells) has endorsements (Fila, Gap, etc.) the whole nine. Don't allow the fact that he's knee deep in tax debt obscure the fact that he's paid, he'd only OWE that much if he WAS paid.
As far as sub-labels and imprints. NONE of those labels have what it takes to sign a legend like Nas. MMG, Young Money, Cash Money, NONE of them could sniff his boots. The ONLY label that would even seem to be a fit musically and professionally would be GOOD Music, and on Def Jam he already has access to Kanye West/NO I.D. Beats if he wants them. Damn near ANY producer in the game would give their left nut to be on a Nas album. No imprint could do him justice....