What's a Triforce of Darkness? O_o
I haven't really be following this RP too closely, but here's my two cents:
It'd probably be best to take place in an alternate reality so to speak. Not like Termina. I mean...during an unexplored time in the series. I don't think you're going to have anyone play Link, Zelda, and Ganon are you? Having someone in charge of three, possibly creating scenarios, and maintaining the order of RP would make the most sense.
Take for instance, the time during which Link's Awakening takes place. Link is out of the picture. He's off in Koholint (and possible Labrynna and Holodrum depending on how you view the timeline). Hyrule is left without their hero. The brotherhood of underworld monsters is trying to revive their Dark Lord, Ganon. Hyrule is in a renaissance.
There's plenty to play with between that time and the time of the original two games - where Hyrule is barren and dry. Hyrule is rising, only to fall. Ganon has fallen, only to be revived. His attempt of creation a Dark World has failed. His minions remain. Link is elsewhere, leaving all the RPers with the chance to be heros. Knights, Sages, Maidens, Royalty. The Triforce is united at this point and Hyrule is prosperous. At some point between ALttP and the original LoZ, the Triforce is split up once again - a wonderful time period for you guys to explore.