I saw scans of some new LEGO Avengers toy which had the "mystery villains" prominently pictured on the front of the box. AND YOU STILL CAN'T TELL WHAT THE $%^& THEY ARE!
Top 4 theories from around the Interwebz:
1) Skrulls
2) Kree
3) Inhumans
4) Atlanteans
I hope they go with #2, it's the only one not lifted right out of the pages of the Fantastic Four.