In my opinion this theme a little exposed.
Although it is very interesting.
For example, the Fallout in the world, due to the lack of heavy equipment and heavy weapons, the role of fortification.
For this reason, fighting trench will have a character as during the WWI.

With a score to settle with the NCR, the Brotherhood of Steel and the Enclave former soldiers should be hidden to help the Legion of Caesar.
Therefore, in the Legion of Caesar must be evocatus - soldiers of the Brotherhood of Steel and the Enclave avenging NCR.

BOS and Enclave remnants fighting together under the legion flag?
I don't think this scenario would be plausibile, at least not the entire faction, maybe some renegades.
The BOS way of thinking wouldn't allow them to obey to anyone but the elders, in their eyes the legion it's only a group of raiders, a very organized one yes, but still raiders.
The enclave as a faction considers everything outside the enclave as scum. the mojave remnants probably did not love ncr but still they didn't have all this thirst of revenge against it, not whitout a good intervention of the courier