Whats the lategame?

[Imperium]

I finished the game on Ultra (in all the expansions), whats next? I could grind for gear to optimize my pet build but to do what? To kill avatar of mogdrogen or ravager? Since im level 100 and there is no paragon or anything it kills my motivation to fight since feels like a waste of time.

So what do you guys do once you are 100 and won all the campaigns?

[gNuff!~©~gNom3™]

get all skill points+attribute points from quests
get all lore notes
^https://www.grimtools.com/checklist/shrines/normal
do the secret quests and kill the secret bosses
like you mentioned, kill Mogdrogen, Ravager and Callagadra at least once in your lifetime, on Ultimate
get Nemesis with monsters and kill all Nemesis at least once in your lifetime, on Ultimate
clear the roguelike dungeons, on Ultimate - farm them
Clear SR shard 50 minimum for the quest, - then farm SR 65/66 - possibly try push shard 80 once in your lifetime and get death's waystone, on Ultimate
clear Crucible wave 170 at least once in your lifetime, on Gladiator
collect all unique legendaries in the game (purple items/sets with fixed spawn/drops)
^do all of the above in farm mode to gear up future chars faster
level up alt characters/different builds
wait for Zantai to nerf your build - then rebuild it
pick flowers

[Faust Wither]

meet all purple family in shattered realm.

[OrbPlaytime]

- Complete up to L25 Shattered Realms Ultimate for 3 skill points and 1 ability point, it can be done on lower difficulties just for the quest and lore notes completion but miss out on the skill/ability points, I would suggest do this on Ultimate at least for the 1st time.
- Complete The Hidden Path if not done so on all 3 difficulties for 3 skill points/ability point.
- Complete Dalia's Hidden Quest to gain access to Edge of Reality; base game, only on Ultimate.
- Complete Lokarr's Hidden Quest to gain acces to Crucible of the Dead; Ashes of Malmouth, only on Ultimate.
- Complete Crate Hidden Quest to gain annoyance of repeatedly having to travel a portion of Edge of Reality to eventually die repeatedly at Crate super boss :) , only on Ultimate.
- Kill all Nemesis, use Grimtools to identify and track them down
- Kill all Celestials, last being Callagadra and Crate on the list.
- Complete L50 Shattered Realm but can be any difficulty if you do not want to do Ultimate apart from L50; I would suggest to do L1-L25 on Ultimate as it has a few set encounters and still kinda fun, then switch to Normal until L50 (you still get the waystone blueprints) and do L50 in Ultimate.
I usually stay in Ultimate but it becomes a real chore and can burn you out this way even if you do it fast and within time limits, and just do 5 shards at a time then coming out of SR for rewards (best way IMO while going up through the levels as you can take breaks).

Sets designed to farm:
Dark One's Gift, Krieg's Armament, Armor of the Eternal Night, The Vanquisher, Shattered Guardian & Lokkar Spoil

[Troubled]

Slowly making my way further and further into the Shattered Realm.
Steal Warden Krieg's toilet paper.
Debating on doing Crucible before forgetting about it altogether.
Running around murdering things, stealing everything that isn't bolted down, and just being a nuisance to everyone and everything that has a red outline for no reasons whatsoever.
Eat Kymon's Wheaties.
Horde unnecessary gear, move unnecessary gear to a character I may not level up. Wash, rinse, and repeat.
Farming crafting materials so I do not have to run around like a chicken with its head cut off when leveling up another character.
Punch Mogdrogen's cat.
Working towards crafting one of each end game relic because reasons.
Occasionally going after achievements because they're shiny.
Chase after anything that has even a the tiniest chance of dropping the last two pieces I would like to get for my character.
Hit on Byscillia.
Look for any hidden areas I might have missed.
Hunt for the last few One Shot chests that I have not found without looking them up.
Get Korvaak drunk.
Theory craft a build for a new character, try it out, and then realize I either need specific gear to make it work or the gear that I want is locked behind a boss I would rather not fight.
Go back to my level 100 character.
Start a watch and see how fast I can farm a route then try and beat that time.
Knit a sweater.

[Enfild]

Go and create new build. I have like 13 different alts. I actually don't have that many max lvl alts, leveling is fun in its own way.
Why limit yourself by only 1 char? And absence of infinite grind enables you to move on after you reach definite endpoint.

Alternatively you can throw your char against superbosses and other endgame stuff, to see if your build can withstand ultimate test, so there is a reason to optimise you lvl 100 build, but its not the main focus of the game for me.