So you start a new char at the entry level, finish the game, and then what? You evidently can’t reset quests so what do you do? Reset dungeons and run around them? I must be missing something.
Typically you’ll unlock Torment 1 by completing level 20 in the pits (soon to be level 10 when S8 hits). You’ll have various tasks you can go have fun in, Infernal Hordes, NMD, Pits, Helltides, Undercity (expansion only), Dark Citadel (forced co-op expansion only), etc. Then it’s just the long game from there, grinding out gear, pushing your build to as high as it can go (if you want), and just having fun.
If you play on the seasonal realm there are seasonal objectives you can complete for rewards. It’s up to each individual player when they plan to stop playing though. I myself typically go for the seasonal objectives each season and stop shortly after, unless friends need help.
Some people like to push as high as they can go into the Pits, as it caps out at 150, and typically most builds can’t reach that high. Up to you what you want to do though.
Kyov and Cerrigar have a big statue which changes “world” difficulty. Level up to 50 and change to Penitent. Then level up to 60 and go to Cerrigar (Scosglen at the North)
There’s an Obelisk there, you’ll notice folks often standing there. Click on the Obelisk you start playing Pits (defeat 20 to increase difficulty and unlock Torment 1, then do the same until you gather power to unlock Torment 2 when defeating Pit lvl40, e.t.c.)
You havent won the game until you are declared the GOAT on forums so only IGGI has won the game.
No, you’re not missing anything.
Yes, reset dungeons and run around them. 
Welcome to Diablo.
The Loot Hunt is “End-Game” & then from D3 to now D4 - pushing difficulty Tiers is additional End-Game content.
It’s glorified ‘dress-up Barbie’ meets ~ fight the dark, evil demon hamster end boss (
) while running the hamster wheel to fight the to run the wheel to…