My idea is:
-Beginning of 2nd movie Spidey fights Shocker in the first battle. Shocker is attacking a major corporate building entitled "Fisk inc." He webs him up and takes his gauntlets to study. The gauntlets were created by Norman Osbourne, an arms dealer to the U.S. military. Oscorp specializes in creating cutting edge weaponry for the military. Looking in the lab you'd see the glimpses of the glider and various weaponry that the Green Goblin would use. He wants to put the only other arms dealer, Fisk Inc, out of business.
-Max Dillon, hired by Norman, goes to "Fisk Inc." after hours to finish what The Shocker started. Wilson Fisk catches Max in the act and attacks him. during the fight something goes awry and Max turns into Electro, Fisk flees.
-Norman believes he can one up the Fisk corporation by designing a new super soldier formula using Spider-Mans DNA. He hires Electro to take a sample of Spider-Mans blood. Electro and Spiderman fight, Electro knocks Spider-Man out and takes a sample of his blood.
-Norman gives the sample to Dr. Otto Octavious (Heard about but not seen). Doc Ock makes whatever Norman asks in exchange for funding for his research. He gets to work on the formula instantly.
-Spidey finds out the gauntlets were made by Oscorp and goes to confront Norman. Norman reveals his plan about wanting to destroy Fisk Inc and to make a super soldier formula from Spiderman's blood. Electro enters and the two battle inside Oscorp. Norman see's that Electro is on the verge of defeat, he rushes down to the laboratory. Electro is defeated, Spidey goes after Norman. Realizing Spiderman will take the formula and he will be arrested Norman takes the formula. The entrance of the lab explodes as Norman flies by Spider-Man on the glider cackling.
-Movie ends with Norman on the loose. Fisk standing in his newly rebuilt laboratory, on computer screens we see silhouettes of RHINO and the Scorpion. Fisk intends to use them to destroy Oscorp.
So basically my idea uses/sets up a crap ton of villains in a way that I think would work.
You'd have Shocker (cameo), Electro, Kingpin, Doc Ock (introduced), Green Goblin, Rhino, and Scorpion. In the third movie Rhino would be a cameo like Shocker in the beginning fight, Goblin would be the main villain and Scorpion a secondary villain.