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Quote from Catalept

This is an awesome idea, but (assuming they're not already working on something like it), it would cost millions and delay release by months.

It would take some time but it would definitely not cost millions to make. It's only a tower and some trials, re-using models, animations and environments that are already present in the game. Even the cosmetic rewards can be something that currently exists in-game (like that El'Druin one in the picture). Balancing the difficulty would probably take much much longer than designing all of this.

I would absolutely love to see something like this in D3! Post it on the official forums as well, if you haven't already.