With 7 years of experience, Bushiroad is rebooting this game for the better. In those 7 years, there has been ups and downs and a lot of things in between. There were oversights, mistakes and successful attempts at trying to make this game as fun as it can be.
On the top of my head, I could think of various things that went unexpectedly wrong (I think the majority would agree to this too): Legion, Barcgal, and 7C Rush amongst many others.
It doesn't have to be card effects though, could be the way the company handles tournaments, product release, game balance, ban list, etc.
The title says it all. Which do you think were the biggest mistakes in this game's history? Why? What would you change to make it better? What do you hope in this new reboot?
(As a disclaimer, this post has no intention to discredit Bushiroad by any means. Take it as a reflective post to prevent such things from happening again in the future.)