Enclave are gone and no power in the Commonwealth, let them die. Also they don't let people join, they hate wastelanders. It's also not really possible to just shoehorn in a new faction to the main story.
NCR and Legion are mutuall exclusive. You can have one or the other not both. They are also like 2000 miles away.
Yeah, BoS built up their expedition to destory the Synths. Enclave have no reason to be there. BoS spanked them before when the BoS were weaker and the Enclave stronger. Enclave have now been wiped out 4 times, 10 years is not long enough to recover from having 2 bases with high value assets destroyed.
Changing their ideology means they are no longer the Enclave.
Depending on how the battle goes, it's still too far for either of them to have reached Boston.
-NCR win the battle they will then spend the next few years trying to consolidate and wipe out the Legion.
-Legion win, they go west and continue trying to wipe out the NCR.
-NV independent NCR is stuck to the West and Legion stuck.
There's also all the other factions like the MW-BoS they'd have to fight through to get to the East coast and then all those inbetween. Neither Legion or NCR can win a fight against the other 1v1 they couldn't manage that.
NCR founded 2196. Finds the Dam 2277. So took them 78 years to get from the coast to Vegas. Second battle 2281, how do you think they would be able to get from there to Boston in 6 years?
It's unlikely NCR or the Legion would cross the Mississippi river. NCR was over-extended just holding the Hoover damn and the Legion has always looked West for expansion.
I'd like to see the Enclave return because of their armor/clothing sets--they look very cool. I'd also like to see a Slaver faction, a deathclaw companion (like the smaller, intelligent one from F2), and more major settlements--the Commonwealth seems rather empty. Skyrim had lots of major settlements as quest hubs. I know Fallout is post apocalyptic = fewer settlements but F1, F2, and F3 had more settlements (I need some more quests/NPCs to bother)
I would like a crossbow to be made available as a weapon. You know, like a modern one of course, and the player be able to make specialized bolts for it. Bolts that pierce armor (and bone), bolts that carry poison, bolts that explode. Some of that may sound a bit like the syringer, yes, but the bolts would deal actual damage when they hit, not just deliver the harmful substance within.
Also, how about DLC letting us create ammo? That was we aren't always stuck with buying it. Would require a certain perk level in the appropriate perk. Like if you want to make the 2mm electromagnetic rounds for the Gauss Rifle, then you would need 4 science AND 4 gun nut.
How about a power armor upgrade for your arm pieces that completely removes recoil? Call it "pneumatic joint locks", and it just completely removes recoil, but obviously takes armorer and science of 3 or 4, since otherwise that would be pretty ridiculous.
Small ideas and changes, and not big DLCs I know, but I think they'd be interesting additions. Also, could we please get a long barrel for the plasma rifle that ISN'T a sniper barrel and still has a respectable ammo capacity and firing rate? And on the topic of energy weapons, why do the laser weapons have any recoil at all? A beam of light shouldn't have any kick... Can we fix that in DLC some time maybe?
Additionally, I feel like the 50 calibre sniper rifle should hit harder than it does. The .50 rounds should be a lot less common (they show up so early and often!!) but they should pack a MASSIVE punch. Like, 'hits you in the neck, blows off your head and rips out your spine' levels of kinetic energy. OH! On that topic, how about a final upgrade to the .44 revolver, allowing us to upgrade the receiver to a .50 receiver? 50 calibre revolvers exist, and I think it would be nice to have a hand cannon like that to pull out on a gunslinger build when you really just want to clear the room.