I've got one thing to say, if T-45d is in the game. I want a damn good reason for it. In the current lore the only store of T-45d they had was located in the Pentagon, meaning there isn't any on the West Coast, unless it was brought there by someone. Only possible way I can see to get your hands on it would be a secret pre-war bunker filled with the junk.
Not exactly true, the Lyons detachment had procured T-45d before going to Washington. They used it during the Scouring of the Pitt, hence why the Tribal PA and Ashur's PA from the Pitt are based on the T-45d and not T-51b.
So they had to have procured their T-45d before then. Likely, they were sent out with only the higher ranks having T-51, and most people having Combat armor (Brotherhood Armor). When they discovered the T-45d, in an undisclosed location, could be anywhere, they augmented their ranks with it, hence the T-45d in the Pitt. When they got to the Pentagon, they found a huge cache of the things in the basemant along with the robot, and decided it would be a good idea to give everyone power armor since they had so much now. Over time all their T-51bs were taken apart and cannibalized to repair the T-45d. (You can even do this in the game with your T-51b if you want.)
So really, we don't know where they found T-45d. It could be they found it in New Vegas even, we don't know. The fact that they haven't shown it off yet is sort of telling, though... I seem to remember one of the legion armors having a T-45d pauldron stuck to it- that pauldron had to come from somewhere.
I doubt the Brotherhood will use T-45d in New Vegas though, even if it's in the game, since they'll have many more superior T-51b models to work with.
What I want to know though, is why the Brotherhood didn't use the Enclave's demise to equip their troops with APA, or at least update their armors with APA design features. It would be nice to see the Knights cook up some Advanced T-51.