I need to get back into the game

I finished the main game quickly after it first launched. But due to having lots of bugs back then I said id wait until it was polished in order to get back into it and go for side quests and whatnot.

Fast forward to today. Game is running great and I really need to get back into it. But I've forgotten everything and don't know how to start. Should I load my last save and explore the main game? Should I start the Phantom Liberty Update? What about the mechanics how do I rediscover the gameplay without touching into spoilers or things I haven't yet explored. Gimme a hand here guys