I was thinking maybe the Brotherhood of Steel are regulators of Boston and Todd's character blew something up like the end of the trailer! Then they started hunting his character down due to his actions and suddenly became a foe!

Probably killed one of them, and they got mad.

most likely that is one of the branching aspects of the main quest, do you ally yourself with the BoS, or do you decide to destroy them.

I don't think the BoS in Boston act as peacekeepers and law bringers. It's much more reasonable and logical to assume the PC killed a BoS member and the other soldiers are retaliating.

Most likely to show they can be enemies this time around. We (probably) won't see them return as the white knights of wasteland this time around but its for the best as they were never suppose to care about people to being with.

EDIT also faction system likes NV's might be implemented.

As said... Brotherhood of Steel could just be the lawmen of Boston...
like The Desert Rangers are the Lawmen of Nevada and Arizona.

well I don′t remeber were, and it′s very vague so take it with a fist of salt, but I do remember a promotional tagline for pre ordering the game mention working for factions.

Its on amazon the promotion page for the game. Its different from NV's description so im sure beth had a hand in writing it up. So we might see it buts an at this point.

It isn't the BoS's job to be the lawmen of boston, its their job to scout for pre-war advanced tech, then go and take it, by force if necessary, and then bring it back to their HQ.

I'm looking forward to massacring the BOS... It'll be a blast.

At least on my first play through...

That's not the job of the Midwestern BoS or the Capitol Wasteland BoS. Only the West Coast BoS and Brotherhood Outcasts remain committed to the acquisition and preservation of advanced technology. The Midwestern BoS was exiled to the East because its members believed in sharing technology to help combat threats such as the Master's army. The Capitol Wasteland BoS is still technically recognized by the Lost Hills elders, but receives no support due to Elder Lyons' shifted priorities.

My theory on the Brotherhood of Steel is that they are like the Western BoS and the Outcasts, in that they're trying to preserve tech. Now, what does this mean for their relationship to the Institute? Unlike some who suggested that they are enemies, I actually think the two are allies. The Brotherhood gathers Old World Tech and tries to salvage it while the Institute makes brand new technologies (like the airship), though apparently the airship doesn't need to be made by the Institute since MidWest BoS have airships.

Except that hard-line BoS would never team up with anyone. They wouldn't be buddies with the institute, they'd decide that they needed to be relieved of their trinkets.

Fair enough. I just thought that having BoS allied with the Institute (who are probably going to be antagonists) would be a good contrast to the CW "white knights" BoS, and would explain why the Sole Survivor was fighting them in the E3 Press Conference footage. Though the explanation that they're really the MidWest BoS (and less "hard-line") could work as well.

Remember... we have joinable factions this time. So there is most likely not one 'antagonist'. You can have the BoS as ally (we see pictures for this too) or as enemy.

My theory is that the BOS and the Institute are at war, and it seems like you will be able to join either side.

As for which BOS division, it seems to be the Mid Western BOS due to the appearance of the airships. Though it might have be a combination of West Coast\Out Cast and MW BOS working together.

The airship which is named after King Arthur's ship? That sounds more like Eastern BoS with Arthur Maxson. Otherwise the name of the airship would be very strange.

Sure, push the faction even further from what they actually are according to the lore.

Regulators of Boston? I dunno, that's not part of their original job description.
If it's members of the "deviant" East Coast Brotherhood I can see that happen, but if they return to their roots at all, or if it is part of the traditional Brotherhood, I don't see them caring at all about the people of Boston.

It all depends WHO these Brotherhood are.

Are they Lyon's group?

The Outcasts?

Midwestern?

Or did another expedition make it out of the Core Region and head East?

I almost wonder if it won't be the latter, seeing as how we evidently bring down an airship of the Brotherhood's, and I can't see the Lyon's group bothering building something like that just to hop over to Boston.