Clothes are the most likely I think, at least starting out.
They probably won't go too crazy with the pokemon dlc. I could see it mainly being event legendaries being given out free as dlc (I think it would be outrageous to have to spend money to complete the pokedex) and maybe they will have a Riolu/Lucario or Zorua/Zoroark type pokemon family as dlc.
There might be some location dlc too and some additional challenges and change dlc (like adding re-battleable gym leaders if it's not in the base game)
I'm not expecting much in terms of dlc until gen 7 to be honest.
If they create package dlc that contains new locations, challenges and clothes I would be more likely to buy them....but I can't really see myself buying single clothes dlc. (Something like if they made a new snowy dlc location with new challenges and features and added in a few winter coats clothes dlc as part of that package)