I agree. Aragorn's story seems to be the most compelling. Teaming Aragorn up with Gandalf for the siege of Dol Guldur would be the way to go.

In musing over "Bridge Movie" possibilities--the White Council's actions don't have to be solely dependent on Saruman--in fact, to show his treachery, it would make sense for their attack on Dol Guldur to be pushed by Gandalf, and weakly supported by Saruman, (or resisted throughout), but accomplished primarily via Galadriel and Elrond (and Gandalf), using the power of the Three Rings (whatever that power is, and however it could be "used"). We should also remember other "powers": Radagast the Brown, Thranduil, not to mention the Rivendell and Lothlorien elves--Celeborn; Glorfindel, etc. The danger is that there are too many secondary characters, and keeping the story tight would be a challenge! (Cirdan could help Arragon protect the Shire for example, in one scenario)
Very interesting!

I am embarrassed to say that I am a novice who recently discovered the prequels of Silmarillion, but more recently I have read The Hobbit again and pondered the Necromancer. My impression from the readings was that Tolkien left it someone vague as him being a Dark Wizard driven out of Dol Guldur near Mirkwood. After Bilbo's adventures there seems a 20 year gap where he frequents the Elves. During this period it could be discovered that the Necromancer was indeed Saulron (returned to Moroor)revealing his plans during his 100 years as the Necromancer. Once in Mordor, Saulron discovers the Rings locaton (and Gandalf) in the shire from the Orcs of the Misty Mountain (via Gollum)and activates it's power through his desire to obtain it. During the 20 years before Frodo you could bridge Aragorns early history , introduce and provide some back drop for Gimli, Legolas,Arewan, Gandalf (and the Belgrod Demon ), Necromancer, Quest against Smaug and restoring the Kingdoms and Peace of Men, Dwarfs and Elves in the Lands of the Lonely Mountain. Prequel Gandalf's destiny to become the White Wizard by guiding the Fellowship and the greater plan for uniting the peoples of Middle Earth into the 3rd Age. This would be a good segway into the Silmarillion prequels I think.