I’m 65 hours into this and only have a vague idea what the main quest line is. I see something on the map and run to it. It’s fun to explore and there is so much. I’ve met a lot of random people that want me to do stuff. My only want in the game is a log of who I’ve talked to and what they said.
It’s very easy to find random npcs. It’s incredibly difficult to find them again once they move. Some of them have natural points where you’re likely to encounter them progressing through the game normally, but only if you’ve already encountered all of their previous meeting points by that time.
I’m not clear what you mean by outside the main quest line. The main quest line is the only noteworthy content. Everything can be done at a later level and probably will unless you’re using a guide to find everything in order. Malenia and mohg are outside the main quest line, technically I guess.