the list below that is people who COULD be in the next dlc.
Lone Wanderer - not likely because every player has thier own LW, and he/she does have a home in the Capital Wasteland, either with Megaton or the EBoS...or both
Harden Simms - still a child in 2281...his father Lucas has too many ties to Megaton and the CW for it to be likely for him to leave
Maggi - still a child in 2281, Billy would never put her at risk by taking her out on the road.
Moira - possible but unlikely, she's settled and has a good living where she is.
Mr. Crowley (After the Ft. Constantine quest he ran off and said he would probably never see you again) - assuming he lives in canon.....most players cheat him out of the armor and/or kill him.
Uncle Leo -possible, he likes to wander and only well armed and organized badguys are goingto be likely to attack him.
Col Autumn - if he survives the purifier in Canon a good bet...he has skills that someone would pay for, he could also form a mercenary group of his own out of what Enclave soldiers are still willing to follow him.
3 Dog - why would he leave his radio station? GNR is clearly a labor of love for him.
Elliot Tercorien - was rather uninterested in the post-war world in MZ
Somah - possible but how many people are going to be pleased to see a slaver from Paradise Falls?
Sally - a child in 2281
Paulson - possible
Desmond Lockheart - strong possibility, he's popular and has a reason to pop up anywhere a pre-war holdout is likely to be. Definitely in the top three possibles
Amata - She's Overseer of Vault 101, unless the Springvale area is completely pacified by Megaton or the BoS she probably will never set foot outside the Vault.
Nadine - Unless part of Latin America has gone missing so she doesn't have to try the passage via Tierra del Fuego, she'd never be able to make it to the Pacific in that boat, especially by herself
Kenny - child in 2281
Charon - outside possibility...but if the LW was not employing him he would probably stay in Underworld to protect it.
Harold (Just because) (Yeah I know hes a tree) he isn't going anywhere
I'd add a couple
Dr Li - she said she was going to the Commonwealth but as Ashur said "In the Wasteland things often don't turn out the way we'd like them to". She could have been refused admittance, got there and decided to leave for some reason, or never made it. If she was captured by slavers, if they have half a brain they'd realize she was worth big caps to the right buyer...which in the Northeastern US would be Ashur. She could probably bargain for her freedom with him and eventually migrate west from the Pitt after the cure was found.
Wehrner- Not popular, but after the Pitt he'll probably be on the run.
Casdin or another named Outcast - with no means of getting in contact with Lost Hills Casdin may give up the Mission when it becomes plain the EBoS not only is going to succeed but irrevocably cast it's lot with the people of the Capital Wasteland and enter it's embryonic government and take his followers back West....or at least send a mission to try to get Lost Hills to send reinforcements and help him depose Lyons. How he/they will react to the inevitable disappointment of learning how much trouble the "real" BoS is in is anyone's guess.