bookworm, I would like to see how Galadriel and the Galadhrim would fare against the Balrog. Note that the Balrog was there (possibly sleeping at times) since the War of Wrath. If it had an itching to take on Galadriel, then it had more than enough opportunities to do so, but evidence shows that it did not. On the another thread it was discussed regarding what the Balrog would have done if it had crossed the Bridge - pursue FOTR beyond Moria or halt at the first gate, content with driving the FOTR out.
Also note that Sauron never overthrew Lothlorien, and would need the Ring to be assured of victory against Galadriel.
Glirdy and
Aragorn II: Aragorn need not die with Boromir, as he did not in the original story. Assume that Boromir comes back after his fight with Frodo, and when everyone goes crazy, Aragorn and Boromir again split. One dies, the other gets to go to Rohan.
Again assume somehow Aragorn can replace Gandalf's role in Rohan/Orthanc. Theoden/Eomer/Eowyn/Merry (and now Pippin) head for Minas Tirith while Aragorn/Gimli/Legolas do the same via Paths of the Dead.
One might think that the story could change in that Pippin is now a knight of Rohan and somehow like Merry accompanies the Rohirrim on their journey. As he then would not be a servant of Denethor (and Gandalf's with Frodo), we wouldn't get to peak in on what the Steward was doing.
At the Battle of the Pelennor Fields, Merry and Pippin could aid Eowyn against the Witch King (assume that as he was going to enter the city the horns blow, and he goes off to fight the Rohirrim irregardless of Gandalf's presence - as some posters believe). This, however, would mean that we would lose Faramir - no one covers his retreat, but even if he survives that his dad burns him alive - which is bad for Eowyn unless she then took a liking for Imrahil.
Or...
Gandalf, when traveling with Frodo and Sam, sees the Witch King leave Minas Morgul, tells Frodo that he is needed elsewhere, and so proceeds the Army of Morgul Vale to Gondor. He could still save Faramir's retreat, face off against the WK at the Gate, but without Pippin to stop him, he would pursue the WK, possibly saving Theoden but Faramir gets cooked by his father.
Unless...Beregond, in a completely nutty moment, leaves his post and gets word to Gandalf, who then saves Faramir.
And as for Gollum - as in Moria, he is going to pursue the Ring wherever it goes, whether overtly or covertly, and so irregardless of Gandalf, Gollum will be there at the end.
This is getting so speculative that my brain is starting to ache...