What's next?
You have finished the game once, completed as many quest lines as possible, and collected all items you could. Is there any replay value in the game? Here are some suggestions:
Obtain all Steam achievements. The most tedious achievements are those involving covenant items. Stay in your current game cycle; don't proceed to NG+1. Farm 30 of each covenant item and turn them in at the respective covenant to receive your rewards. Refer to the guide below. Various other achievements require you to play through the game a few more times. In total, you need only to play through the game up to and including NG+2 to collect all achievements.
https://steamcommunity.com/sharedfiles/filedetails/?id=2081313800
Speedrun the game multiple times to reach the maximum soul level of SL802. Refer to this guide:
https://steamcommunity.com/sharedfiles/filedetails/?id=2147255341
Mod your game. WARNING: modding your game and playing online would likely result in your Steam account being banned. Only install a mod and play modded DS3 if you know what you're doing. As a start, you might want to experiment with Cinders mod and/or Convergence mod.
- PvP and constant invasion. Anyone who wants to play online should install Blue Sentinel mod to protect themselves from cheaters. For invasion, use Wex Dust mod to help you quickly find a host to invade.
Do challenge runs. Examples include:
SL1 run.
How to Survive Your First SL1 Run in Dark Souls 3
How to Survive Your First SL1 Run in Dark Souls 3 - DLC
https://www.reddit.com/r/darksouls3/comments/4g4wy9/dark_souls_3_sl1_guide
Speedrun.
Class challenge (EpicNameBro run).
https://www.reddit.com/r/darksouls3/comments/4jdkee/class_challenge_revisited/
Gandhi/pacifist/passive run.
https://www.youtube.com/playlist?list=PLRI9RAaDGpwm6NqiGTg_un32eUM5AbY28
No-hit run (Happy Hob run).
https://www.youtube.com/channel/UC0E1n0GRgBW5gR7y7H9TjZQ/videos
No rolling, blocking, parrying run (TolomeoR run).
Fight bosses with consumables only.
Wear Kirk's armour set and fight bosses by rolling into them.
https://www.youtube.com/playlist?list=PLVDIXUXJIIvHuKNXFu5zrw8_oVzX8XALF
Archer run: fight bosses with bow and arrows only.
Dragon boxer run.
https://www.youtube.com/playlist?list=PLbgNBlGYFN-OoYEvdzKiHVSNxpd8G4H2a
Cheese run: fight bosses using cheese tactics only.
DS3 Glitch/cheese all bosses in new game challenge! ( no outside help and on console! )
See this page for more inspiration: