I was level 119 when I finished the main quest and I am now at level 190. I've spent the time building up the settlements, exploring and gathering resources, and every week or so I go on a tour of the heavily populated/high level enemy locations to level up and try new weapons and perks. The game is not a challenge in that I'm in any danger of dying but I like the challenge of clearing areas without being spotted or taking any damage. And yes I would play a quest mod set after the end of the main quest. New quests are what I miss at high levels.