So, what happens after Act V? [duplicate]

That is intentional. You are supposed to be around level 38-42 when you beat the final boss. After that's done, it's up to Adventure Mode to rise that level (for the most part).

What exactly is Adventure Mode?

After you’ve completed Act V and beat the game in normal mode you’ll have enabled something called adventure mode. Adventure mode let’s you pick and choose the difficulty and area you want to farm. It also has bonus missions like bounties and nephalem rifts.

Now you can repeat lots of previous Acts with a twist to grind gear and bounties!

In addition to Adventure Mode, you can still grind previous Acts but killing every monster, elite, looting every chest etc. in hopes of getting legendary gear to improve your stats greatly!

If you want a challenge, you can modify the difficulty of the game, the higher the better the loot rewards!

If you want an even bigger challenge than you can create a new character designated in Hardcore mode! This Hardcore mode is really brutal, you have only one life and if you die you lose your whole character. You cannot reverse the Hardcore mode. More information can be found here.

That pretty much sums up Diablo 3's endgame. Grinding. Lots of it.

There is one major side quest at the Whimshyre, which is Diablo 3's secret level. The main goal is to craft a level 70 hellfire ring.

Here's how to get to the secret level: click me

Here's a great tutorial on what you have to do at Whimshyre, and how to get there:

Consider a trip to Whimsyshire and craft a level 70 hellfire ring

Diablo III featured two main quests to find when you'd reached the end of the game: Whimsyshire and the ongoing toil of forging a hellfire ring. These have effectively been superseded by Adventure mode, but it's worth keeping them in mind if you're the kind of person who wants everything.

Whimsyshire is Diablo III's secret level, where instead of fighting demons and monsters, you're pitted against unicorns, cuddly bears, and flowers in an environment with rainbows and happy bouncing clouds with smiley faces. How to get there? First you'll need the blacksmith plans to craft the staff of herding, which drops from Izual in Act IV. Then you'll need to collect five rare items: the black mushroom from the cathedral and Leoric's shinbone from Leoric's Manor in Act I; Wirt's bell from town vendor Squirt and liquid rainbow from the mysterious cave in the Dalghur Oasis in Act 2; and, finally, the random gibbering gemstone drop from rare monster Chiltara in the Caverns of Frost in Act III.

Once you collect all that (it'll take some time and multiple attempts, especially finding the gibbering gemstone), you can forge the staff of herding from Haedrig and then head to the now-glowing crevasse on Old Tristram Road in Act I.

A hellfire ring is also seriously hard work. You need to farm four powerful keywarden enemies, over and over and over, with one in each act: Odeg in Act I's Fields of Misery drops the key of bones; Sokahr in Act 2's Dalghur Oasis drops the key of gluttony; Xah'Rith in Act III's Stonefort drops the key of war; and Nakarat in Act IV's Silver Spire drops the key of evil. Each of the keywardens can also drop the plans for the four infernal machines, which can be given to the blacksmith for crafting.

The catch? You have to be in at least torment I to be eligible for key drops. On torment I, there is about a 25 percent chance of a key dropping. On Torment VI, there is about a 50 percent chance. It's going to take you ages.

These four keys can be used to build one of four machines, which can be used to enter the four challenging boss fights, with each boss having a slight, unknown chance to drop an item that can be used to craft the hellfire ring. If the boss doesn't drop the item, you'll need to farm the keywardens again until you can build another portal to give it another go. Like I said, it's seriously hard work. Oh, and you need to buy the plans from Squirt in Act 2 for 5 million gold.

If you manage to go through all of that, you'll end up with a level 70 hellfire ring, which has no level requirement, has five random magical properties, can cast a ring of fire that causes 200 percent weapon damage, and grants 45 percent bonus experience. It doesn't really end up being worth it, but it sure is satisfying to finish the job. And the experience bonus is incredibly useful for levelling up alternate characters.

In addition to all that, this is a great list of other potential endgame goals you can do in Diablo 3.

Source