Hey mate, I wouldn't worry about the season pass. It's way too expensive and lately all the DLC is crap. Well, it's not "crap" I suppose, but it doesn't seem very enticing to want to come back. It's mostly just skins and stuff.
In regards to Arkham Origins, I agree, that did have a really good story. I think it's plot is on par with Arkham City, actually. Although to be honest I do prefer City's plot.
They are releasing a multi character challenge mode thingo called the "Crime Fighter Challenge Pack #1", which allows you to play as Robin, Nightwing or Catwoman in those virtual simulation training things. Since it's being called "#1", it's safe to assume that they'll probably release two or more Crime Fighter Challenge Packs.
Personally, I think that's "crap" because this is something standard in all Arkham games and it should have been released to begin with in Arkham Knight, not followed up as DLC we have to pay for.
They are bringing out a new story mode pack called "GCPD Lock Down" that's starring Nightwing. Not much has been revealed, but the basic story of this DLC takes place after the main campaign of Arkham Knight, and it's about the Penguin trying to escape GCPD. I might play it, but if it's anything like the Batgirl DLC, it'll probably be wiped clean in about 90 minutes or less and will probably be over priced.