So what is that secret combination of what we will be seeing sometime later this year? Well over at the BOLS forums the following has been suggested... Dark Eldar, Eldar, and Tau. Its wrong. Lets take a look at what the rumor mongers say.

First off...
As for contents of the second flier wave, the guess over at the BoLS lounge is the Void Raven, a Tau flier, and an Eldar flier.


via Harry
There is another wave of flyers. ...
Thats not it.
 
Notice he gives nothing else. So lets do a little digging to hopefully shed some light on the subject.....
 
Dark Eldar (via MajorWesJansen)
Void Raven, it is not the same as the Razorwing, but is larger, 4 engines, a nose canopy, forward canards. I've seen the concept art of it and discussed how to break it down into sprues with Jes himself. So it's not going to be a FW conversion kit, but a planned plastic kit that is somewhere in the pipeline.

Tau
Likely to come out with their codex. They are rumored to be the first codex of 2013. Likely February.

Eldar
Its possible to see this vehicle in the 2nd wave, although its looking like its quite possible to be given to us alongside a codex release further along in 2013. Probably a 50/50 shot at the moment.

Other Codices
Imperial Guard may very well get their Hydra in the second wave
Tyranids- Are looking at the Harpy, and possibly a new monstrous creature (dual kit?)

Chaos
Dragon like Daemon Engine is rumored, so I expect this to cover both Chaos Daemons and Chaos Space Marines. This guy is expected to be released as part of two quick waves of Chaos Marines.

So when is this 2nd wave of Flyers going to arrive?
First month available appears to be August, followed by October or November. However I am going to throw down what I believe the rumor mill is hinting at......

July: 6th Edition: Rules (Rules for Flyers included in Std Games)
August: Chaos Marines + Wave of Chaos Models
September: 6th Edition Starter Set followed by a  Chaos Marine Wave ( finecast)
October: 2nd Wave of Flyers including........ Voidraven, Harpy dual kit, Hydra Flak Tank

Then its all Hobbit Hobbit Hobbit until February's Tau codex release.