The Warhammer may be in the game. It was seen in the old, outdated trailer and briefly mentioned in this Q&A, in which answering a question about it seems purposely avoided.
The Warhammer has been announced as a special pack of its own, like the UrbanMech and the Marauder.
The Warhammer was one of the 'Mechs featured in the MechWarrior reboot trailer from '09. The appearance of the Warhammer is in doubt since both the original version and its reseen redesign have both been considered unseen designs now. Below are both the early and final versions of the final trailer.