Mark my words, it won't be a shield button. It will be a shield grenade, a 4th type of grenade for the game, to make the total number of grenades you can carry back up to 8. And my prediction is that you can use it not only on yourself, but also on your team mates to provide protection for them.
I also predict that the X button will be for a completely different feature that they may not reveal until close to, or even until the game is released.





Bookmarks