1. You don't start with the wonders.
2. Thats just for one battle before needing to return so not a huge advantage anyway. And unless you know what deck your opponent will be using as well, the advantage is even smaller.
2. Its not really more powerful then the others.
As for your suggestion its still really only tenuously related to darkness at best. And the underworld is still far more closely related to death then darkness. Especially going by Elements definitions.
And Underworld Movie Series is 100% unrelated to this.
1. Even if you don't start with it, your elements' wonder would most probably be close to the Water city because of the small map size. And theoretically, you don't have to be the one to claim the wonder. As in Heroes of Might & Magic, once the NPC guardian is defeated the Wonder is claimed for all heroes of the same element.
2. Giving a player all cards or an extra life is no small advantage. One battle is worth at least 12 lost cards and a lot of action points depending on where the battle took place. Such an advantage is too much. Also, balance between the wonders would be harder to achieve with powerful abilities.
3. The Underworld is definitely not all about dead. Like Hell is not all about dead.
4. Until they are actually used or accepted, a lot of ideas here, if not all of them, are technically "unrelated" to this game.
I get it that you don't like my idea as much as yours. But it's the same on my end. You'll just have to learn to accept it, wait for the organizer's decision, and hope for yours to get chosen.