My ending: Scratch the wedding scene -- after all that has happened, a girl as smart as MJ would never have come that close to going through with it anyway. I would have MJ give her "I'll decide what risks I can take" speech while she and Peter are dangling in the web he weaved after her rescue from Doc Ock. When she finishes insisting that she can handle the danger of being Spider-Man's girlfriend, Peter whips a web around her waist, pulls her to him and kisses her (which is certainly no more of a cliché than a "left at the altar" scene)...and as the camera pulls away...
...fade to the scene of Harry losing it in his penthouse, smashing the mirror and discovering his father's Green Goblin lair. End on that, and you've got a lot more instant anticipation for Part 3, plus you don't have the awkwardness of seeing Harry at the wedding looking all calm and normal.
But that's just me. I could be wrong.