Tau and Necrons are still a long way off, so no one go off the deep end in excitement. I have done what vetting I can on these, but it is way too early to not have loads of salt on hand.
Tau
New Heavy support platform. Basically immobile deep striked! heavy weapon and crew. Rail and Ion Cannon, and missile launcher are all obvious options here, but nothing solid with the rumor.
Assault Skimmer. Something between size of piranha and hammerhead, fast skimmer open topped, assault on disembark.
Update: Yes, its assault after a flat out move, and has the "drop troops along path" option similar to the storm raven. its supposedly a dedicated transport option for one new unit. (I dont place faith in this last bit, its very rare in current GW systems to see a vehicle kit limited to one unit/role...if GW is trying to sell them they will be available to more than one unit, or be a separate entry unto itself.)
New Drop ship. Not the FW one, but something new. Holds a boatload of fire warriors, or a few battle suits.
New battle suits. Not sure if these are the "next gen" suits or just heavy suits redone with out metal bits.
Necrons
All Immortal based models being redone. Because Destroyers, Lords, etc. were supposedly based on the immortal "chassis" and that has been redone "bigger", its a cascade effect. However, skimmer body base not expect to change.
Immortals possible move to Troops choice.
New Elite options
Pariahs no longer 0-1
Additional C'Tan
Update: This may not actually mean what it sounds like.
plastic Tomb Spyder box to make TS or variant.
New fast attack MC
Pylon may make codex as heavy option
Space Marines
Update: Another interesting rumor that I got back was a new SM kit has been started...similar to the sm captain plastic kit, a SM librarian/chaplain character box. This makes a lot of sense as the existing line of generic librarians/chaplains fail to encompass many of the wargear options in the marine codexes. I see this as a very real possibility, but its a very raw rumor, so it could be a long ways off yet. And instead of a 10 metal blister they can sell a 15-20 box of plastic.