Wait.. you consider the New Vegas Brotherhood to be radicals? To me it seemed that aside from Veronica and Elijah they were such traditionalists that it was literally killing them. The lack of recruiting and adapting to new ways had reduced them to a handful of members who locked themselves into a bunker. Even the Outcasts from Fallout 3 were better off. Lyon's Brotherhood was obviously a radical splinter faction.
To me, 'traditionalist' means 'radical', at least in terms of extra-Brotherhood politics
Regarding the summit, that's why I specified 'progressive' factions. There are groups, such as the remains of the Enclave and traditionalist Brotherhood chapters that would never be involved in it. Others, like House, would be very much only interested in this if it benefited them personally. I'd expect the NCR to have an attitude much like the Enclave's - We'll play but only if we get to be king.
However, although those groups are powerful, that doesn't change the fact that the independent and other rebuilding-minded groups are powerful in their own ways and, if they pooled resources, would be able to overcome issues like long-distance communications. The Couriers, for example, might be willing to extend their routes eastward if the East Coast factions could guarantee a degree of stability so that their people aren't fighting every step of the way. Then there are the crazies at Nellis who might be able to reinstate cross-continent air links if the Arroyo tribesfolk were able to restore the fuel production in their area with Brotherhood, Think Tank or NCR assistance.
However, I would think that the whole point of a 'Grand Summit' DLC would be for the player charater and allies basically having to identify and choke off plots from the Enclave Remnant, traditionalist Brotherhood chapters, radical imperialist factions with the NCR. Then there are the likely overt attempts to destroy the conference by the Enclave and whatever is left of the Legion.
Ron's Max Karma 'good' ending narration for that would be interesting to hear.
"
The Sole Survivor, Courier Six and The Lone Wanderer all agreed that they hated the triumphal statue raised intheir honour on the banks of the Mississipi River where they finally and forever turned back the Legion, Enclave and those of the Brotherhood that chose to believe that they were superior to all other men. Frankly, no-one was particularly inclined to listen. Rose of Sharon Cassidy told them to 'fucking suck it up'. "
Joshua Graham, The Burned Man, simply reminded them of the power of symbols in giving hope and direction to the many."