How are they rushing the Sinister Six? They established Lizard in the first film, you have Green Goblin, Electro, and Rhino in this, and all that is left is Norman's Goblin and Doc Ock and Vulture (which I still feel Donald Menken is actually Adrian Toomes).
Smythe could make Green Goblin's new suit. The Man In the Shadows and Norman could be taking turns as the Master Planner and IF Carnage is to appear, he'll be in a Venom movie which "ASM2" is teasing. All you need is either Eddie Brock or Flash Thompson get ahold of the suit and if Felicia is in this movie then she can easily appear as Black Cat in a sequel.
If you ask me, they have things pretty much handled.
However, I do wish Spider-Man was getting the symbiote before anyone else.