Given that the thing I'd want the most, generally speaking, is some addition that would make a new playthrough feel at least a bit different, I'd be temped to say "a new class".
And yet, there's this general problem with D&D that many classes before level 5 can barely feel any different from each other. For instance a Paladin, Ranger or Warrior before level 5 would feel pretty much the same to play, bar a couple of once-per-rest abilities.
All things considered, if I was forced to pick ONE of the three I'd probably go for raising the level cap to 5. It would introduce the largest amount of new content in the form of new spells, class abilities, etc.
A new class would come as second option.
A new race as third. Potentially negotiable according to what race we are talking about. Meaning how exotic and different from the current ones it could be.
