O'Brien coming back just because her new Ladyship (or Lordship) wants to make business with the Crawleys (a trainwail or something crossing the India) yesss! She doesn't need to return as staff, she could be only a companion. Or her employers are runing away because anticolonialism war or something.
Or maybe Thomas's sister is a cinnamon roll, that's why Miss Baxter was friends with. Anyway, it could be interesting to see the dinamics between Thomas-Baxter-Sister who maybe or maybe not knows Thomas' sexuality if
a) Sister is the source of all Evil and Doom
b) She's so adorkable and sugar saccarine that leaves Baxter as the Devil or
c) She's the real bitch in sheep clothing.
d) Whatever option, while Thomas is cold as an iceberg, this sibling is like a volcan exploding, hilarity issues with Carson XD
Headcanon goes for Katie McGrath or Eva Green now XD It piss me off that a character who links two quite main characters and is the main reason for one of them to be at Downton is never shown or discusses about.
Miss BaxterxMolesley and PatmorexMason are the only OTP I'm sure about will work without any problem.
As for Daisy, it is said in the wikia that she has like ten brothers and sisters and a very busy parents, that's why she projects on Mrs. Patmore. But it could be very interesting too to see a woman of her time dealing with the children/family stuff and working and having ambitions on her own, from the point of view of a servant (Edith doesn't count, she has the support and wealth of her family). Also, a bit more of Andy's background, we literally didn't know anything about him except "he's nice", "he lived at the city" and "he didn't know how to read".
And what the hell happened with Alfred-Ivy-Jimmy Kent (especially the late?).