So, I recently finished Skyrim, and I want to get into more Elder Scrolls games, as I wait for Elder Scrolls 6 to come out. Should I play Morrowind or Oblivion? Personally, Morrowind sounds a bit too grindy for me, but I don't know.

I would suggest Oblivion next. I think, in many ways, it is closer in spirit to Skyrim, despite some differences in game mechanics. It is also more graphically advanced than Morrowind, if that is important to you.