Who will be on The Invaders? Johnston wouldn't say, but here are some possibilities, based on the comics:

The Human Torch. No, not that one. The original Human Torch was an android who appeared in Marvel Comics #1 in 1939. At Comic Con Feige confirmed that while Fox owned the Johnny Storm Human Torch from Fantastic Four, Marvel Studios retained the android Torch. And in fact Feige used the android Torch as a way of explaining why the Marvel Universe's WWII is so distinct. Don't be too surprised if the Torch shows up. He had a sidekick named Toro, who had the exact same powers as the Torch. It may be possible that Marvel uses the Toro name to avoid confusion with the Fantastic Four character (and to make the Torch Spanish), but I think it would be great to spit in Fox's eye and use the original Human Torch.

Union Jack. I'd more or less bet money on him being on the team. In the comics, Union Jack is an older adventurer who was active in WWI. He had no superpowers but wore a bulletproof outfit and was exceptionally athletic.

Spitfire. Union Jack's daughter, Spitfire can run at superfast speeds, leaving a trail of fire behind her.

Blazing Skull. A really weird character, Blazing Skull is a guy who trained with Chinese  monks and has superhuman strength and can make his tissue - just his skin and muscles! - invisible, leaving the appearance of a walking skeleton. He wears a flaming skull mask. The Blazing Skull was an American in the comics, but could easily be made into a Chinese character. The only downside is that visually he's identical to Ghost Rider.

Silver Scorpion. Another American who could be changed into a European. A woman who is a jiu-jitsu master, Silver Scorpion had a sting-ray built into her suit. Despite being called the Silver Scorpion she had a yellow costume with a red cape and blue boots. A massive redesign would be needed.

European Defense Initiative. It's also possible that the film will go with a concept floated in The Ultimates, the EDI. Although set in the modern day and not in WWII, EDI is made up of heroes from multiple European countries, with characters like Captain Italy, Captain Spain, etc. To me this would be a cheap and easy cop out, though, and if The Invaders are important in the second half of The First Avenger: Captain America, it would make more sense to create full characters for them rather than simple Captain America knock-offs.

Namor, The Sub-Mariner. Incredibly unlikely, but possible. Fox, holder of the Fantastic Four characters, has Namor, but might that just be the Silver Age version? Namor actually first appeared in Marvel Comics #1, in the same issue as the android Human Torch. He was a founding member of The Invaders (which was actually called The All-Winners Squad back in the 40s), so he really belongs on the team. Of course even if Marvel does own the Golden Age version of Namor (which is essentially exactly the same as the Silver Age version), they may not want to confuse matters by using him in The First Avenger: Captain America while he might yet turn up in a rebooted Fantastic Four or his own film.

Others. There's a whole host of WWII characters that could be used, from The Thin Man - one of the first stretching superheros, beating Plastic Man to the punch - to the Blue Bullet - a Polish Jew. It's possible that the film could dip into this massive, mostly untapped well of characters and really surprise us.