[Kransha]

Well, an alternate ending doesn't necessarily have to drastically alter the storyline, which would be appropriate. Some possibilities could be engineered to alter the moving of the story, but not the fitting ending.

-While Aragorn leads the forces of Gondor at Morannon, Eomer and/or Eowyn lead the remaining Rohirrim and supplementary forces into Mordor via Cirith Ungol or another mountainous entrance. They ride across the abandoned plains of Gorgoroth towards Mount Doom. Perhaps, being on horseback, they catch up with Frodo and Sam. Finally we get to see the One Ring's effect on the people of Rohan.

-Interesting diversion from the story. Have the army that attacks Minas Tirith not contain any of the orcs under the Witch-King at Minas Morgul or Cirith Ungol. After Pelennor Fields, instead of attack the Black Gate, the last army attack Minas Morgul with the Nazgul still around. What effects would that have on the plot?

-Instead of heading to the Shire, Saruman takes the remainder of his armies and all the forces he can muster to rally around Sauron. He goes south instead of north, spreading dissidence, and attacks the mustered Rohirrim at Harrowdale. The Rohirrim could probably win, considering the superior losses, but what if someone important ended up being slain. That could engineer a slightly more adventurous end for Saruman and an interesting plot twist.

more later...