I too am eager for the new IG Codex, I expect we'll see some neat new Orders and various groupings of Orders, kinda like Chapter Tactics. Not so sure about special psychic powers though, seems like IG would be pretty run-of-the-mill with respect to psykers, though the psyker battle squad is pretty cool, and deserves an overhaul somehow (perhaps they'll get some ridiculous Allies-possible bologna combo to be exploited wholeheartedly).
And hopefully a cool new tank kit to make Colossuses (Colossi?) and that sort of thing. Maybe there will be new transports too other than Chimeras or Vendettas? Getting the Vets and Special Weapons to the backfield is key to taking those far objectives.
To deal with FMCs, the Guard can field bunches of small units to force Grounding Tests, and then unload the heavy stuff on them when they get Grounded. If you face Fateweaver and Daemon Princes/Greater Ds, ground the Princes and Ds first, as they will hurt in combat, whereas Fatey hurts NOTHING in combat.
Not sure about Nids though, I think the IG just need to dedicate tons of lascannons/missiles/autocannons at the synapse creatures. A Carnifex was forced to eat itself at my FLGS last weekend. WoW.