What are some of you guys doing after finishing the main quest line?

everything that I can find

I'll probably end up starting a new character and play through with mods and starting off with survival mode instead of waiting until halfway through the storyline. I grabbed a bobblehead early, so I have 10 points in one of my special stats instead of potential permanent 11. Before I start the new character, I'll get all the remaining companion perks, explore every place I haven't yet, and collect every magazine. In my second playthrough, I'll develop the settlements more. Currently, I just add the basic resources, defenses, etc in order to keep my settlers happy. I'll do some actual aesthetics for my second playthrough.

I always leave the final quests as the last thing to do. I never continue after it.

Side-questing, exploring parts of the map I haven't visited yet, settlements, and running through the main quest on two different characters with different factions.

The same things over and over and over..............

Role playing my character for a loooooong time lol. With all they added in this game, that activity will keep me busy for a long time to come. I'm hoping that some of the dlc they release will give us post story based content. For me, completing the main quest is just the beginning. Hopefully the game is very stable because I am going to go all in. I will make another character eventually just to blow through the story and get the other achievements I didn't nab.

I am going to continue to build up my settlements, role play out my romance more, clean up downtown, collect magazines and bobble heads, customize my power armor, explore, maybe find a new permanent base of operations rather than the truck stop, try to rationalize the decisions I made over the course of the main story and come to grips with them, make my ultimate weapons, and on and on.

In Skyrim, the end of Alduin was just the beginning of my adventure each time

Main... quest? Oh that "Go To Diamond City" thing? Still trying to deal with Preston's incessant crying over settlements and exploration.

if I ever get around to finishing the MQ, will probably start over, and not pay much attention to it. maybe bits and pieces here and there, but focus more on exploring and side quests.

The end of the main quest is never the end in Bethesda games. As long as there are new locations to explore, new things to find and new side quests to complete you haven't finished the game yet imho.