Well it has been somewhat discussed in previous threads but we then have to go in on what factions will be presentable, also when digging into that, where is the game supposed to be and in what time.
We can most certainly include raiders at least, they will most likely be against everybody. Depending on where we are we might see NCR, Vault City and San Francisco. BoS might also make an appearance (i think so since so many people connect them to fallout). Slight chance of enclave coming in but that is very debatable.
Well, BoS and NCR don't like each other that much (IIRC they had a war). But I don't think they will risk open warfare (not BoS at least) now. Got no idea what will happen against the other 3 'nations', guess that will have to depend on how fallout 2 ended and
IF the game is in California and IF it is after fallout 2.But those are more like nations, they are also factions but there are ofc smaller ones like the New Reno mobs (or gangs in general), slavers, traders/caravans etc etc and their loyalty can often be logically thought out, (towns that don't like slaving dislike slavers, almost everyone likes traders except raiders and another nation of some sort that want to cut off supply-lines), mobs usually have weak alliances amongst each other, don't attack me and I won't attack you but often don't care about other things that are far away from them, may be have secret alliances with different factions like slavers, drug dealers, porm industry, weapon dealings, etc etc and ofc other factions go via them (the mobs/gangs) to get connected to the other factions so they are not directly in communication with them.
Just look at normal society and there you get a grasp on how it works