@MiyamotoMusashi:
I have also a theory concerning the S.A.D or rather i read the theory in a german forum and just wanted to see what you guys think about it. Before i write it, i should warn you that i have no knowledge of physics whatsoever. If that really is important concerning OP is questionable though:
We already know that the magnetic fields in the NW are even more fucked up than in the Paradise. Generally you can judge the potential of danger of an island with the movement of the Log Pose. Sometimes the magnet field of an island even disappears, that is why the Log Pose for the NW has 3 needles, you can always reach an island with 3 needles. So basically, the unpredictable magnetic fields make the NW an extremely dangerous place (well, besides the Yonkou etc,). We also know at this point that there are certain S.A.D. tankers which were used by Vergo to reach PH. The thing is though, Punk Hazard should not have a magnetic field anymore, so how was he able to reach PH?! That is where the S.A.D. comes to play. The S.A.D. material or whatever it is somehow has the power to shift, erase and create magnetic fields. With that, you can pretty much have a safe trip, at least concerning magnetic fields, and at the same time screw over many others. So, Law´s plan could entail among others the deletion of the magnetic fields and thus messing up the entire New World like Vergo said.
I like this idea. As for Caesar's part in it, he might be needed to actually create the S.A.D. Remember, the guy is a Gas Logia, so he should be able to cause chemical reactions that would normally never be able to take place, and its possible he needs to be there to create as much as Law would need. It would also explain why its on PH in the first place, as it would have to be close to Caesar to be useful. Its possible that Doflo is using it right now (maybe to hide his own island somewhere?), since it looks like the S.A.D. tanker Vergo arrived on could be there for its regular shipment.
But yeah, that sounds like an interesting plot turn. Its a power that could really mess up the way the new world works, and could be used to isolate entire islands for attack, which is a huge strategic advantage if you're taking on Pirates like the Yonkous who hold multiple islands and probably have their strength spread out. Given that Law's power allows for rapid transit between places without needing the log poses (we can assume his Shichibukai position gave him free travel of most of the new world, letting him set up his own "rooms" there for later use).