What is the one thing you want from the first two games into D3?
Synergy. That is my personal pick.
It's quite simply one of my favorite accepts in Diablo II: LoD. Granted, it wasn't added for years until later patches but it changed many classes. Giving rise to some very unique and powerful builds. ( Hammerdin comes to mind )
Runner up would be light radius. I felt that is what made the game feel a bit grittier. Darker in a sense.
Post what you'd want from Diablo or Diablo 2. From lore, story, characters, game play, etc. What you'd like to see added or weaved into D3.
They tried that at some point actually. I think they showed some examples at Blizzcon in either '10 or '11, can't remember which it was. But they dropped it again due to the nature of the skill system. It was impossible to balance around the synergies given how many choices people could make.