Re: things to experience before finishing the last main quest (lots of questions)
1. Exploring after the main quest can still be worth it. Inner circle quests are blocked off, but if you like exploring, improving equipment, and killing dragons, the game is still fun after completion. I strongly recommend the Trespasser DLC. This DLC takes place two years after the main storyline.
2. While I didn't like Sera the first time I played the game, she grew on me and is now my favorite character. She's a bit of a dimwitted, playful nutcase, which is a nice contrast to some of the other characters. Blackwall has a very interesting personal storyline. Keep in mind that these characters must be recruited before leaving Haven, so you'll have to recruit them in the next playthrough.
3. Redcliffe has a griffin statue honoring the Hero of Ferelden, but it's not really worth visiting.
4. Alistair has a small role if he was king and you sided with the mages. Furthermore:
Unfortunately, Sandal is not in the game.
5. Romance continues in the Trespasser DLC, but not in general post-game exploration.
6. I definitely recommend mixing it up. Sera's banter with Solas and Vivienne is great. The banter system is supposed to be timed, though I feel some areas have less banter than others.