I just thought of an answer to my own question from last week.
<BLOCKQUOTE><font size="1" face="Verdana, arial">quote:</font><HR> does anyone know why Sheriden gets to be the One that will be, when he dies in 20 years and Delenn lives to be over 100? <HR></BLOCKQUOTE>
All the stuff about whose turn it is to be Entil'zha makes sense, but it's too short sighted. Sinclair, the One who Was, lead the Rangers 1000 years ago and then he died. But the Rangers still said "we live for the One, we die for the One" before Delenn became Entil'zha. My theory is that their devotion to Sinclair/Valen was literally in a religious sense. Even though their great leader is gone, they still fight for him and his ideals.
Skip ahead 1000 years to B5. Delenn is the chosen of Dukhat, unofficial leader of the religious caste, and wants to personally be in charge of making sure humans and minbarri are together for the next great war. She takes apart the government (set up by Sinclair/Valen) and creates a new one. By the time the Rangers want to declare her Entil'zha, Marcus is ready to live and die for her as the One. (Remember Marcus left the room when Zathras gave his speech about the three Ones.) Delenn obviously holds much power with the Rangers and the whole Minbarri society.
Skip ahead another 1000 years to the monks that we see briefly at the end of season 4 (sorry, I don't remember the title). These guys refer to "the Blessed Sheriden who dissappeared beyond the stars." (If I misremember this part, hear's where you would shoot down my theory) Sheriden's Oneness doesn't refer to his leading of the rangers, it refers to the time in which he Will become a spiritual icon, just like Valen.
I don't know how much sense that makes, but at least there is symmetry.
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Peace be with you