What if scenario:

[hereatlast]

What if Saruman had repented of his evil deeds and joined forces with Rohan and Gondor in the fight against Mordor. Also; instead of defending Osgiliath and losing many lives in the foolish attempt of retaking it, Gondor pulled safely back to within the walls of Minas Tirith. Orthanc marches with their host of Uruk Hai and reinforce Minas Tirith (Along with Saruman himself).

These forces join together in an attempt to hold out against the oncoming horde.

(Boromir has not died, and the Elvin host joins there as well. (Including Elrond, Glorfindel, and any other worthy white-council members/ elves capable of assisting. Aragorn, Gimili, and Legolas are there as well.)

Two different scenarios:

Rohan joins Gondor, and Orthanc forces within the walls, or remain out of sight to "plow" into the side of the Mordor forces after they have set upon the city. (Possibly countering Grond.)

Have at it!