Don't know.. Sandman + Harry would've been enough for one movie, and then save Venom + Harry's death for Spider-man 4.
All I know is, that this should be the end of the current spider-man movie series. You can not top the changes that Peter has gone through, from Discovery, to Responsibily, to Vengeance and Power. There is really little for Peter Parker to go. He and M.J. need to already get married, Harry will be gone, Aunt May is due soon, and the glory that is Venom will sink away.
Spider-man 3 will end on a sad but powerful note. This kind of sadness of death and despair hopefully won't dampen the greatness that is Spider-man's character and series.
Although, I wouldn't mind a fast forward into Peter's future w/ newborn Mayday Parker somewhere down the line. That would be interesting to see kind of the way Superman did it. Agaisnt the cannon and whatever else, it would definetly be a change from the current (Save the girlfriend thing) and more of a save myself and my daughter from evil.