A few ideas on what we could do and maybe do certain things after a player reaches level 100. Also some ideas on how to do certain things at level 100 and beyond.
Equipment and Gear:
-Have a set of gear at level 100, helm, armor, legs, ect... then make a new set of gear for like every 20-30 levels. Make a full set of gear at level 120 and then another set at like level 150. Help spread the gap in gear instead of making a new piece of gear for every single level in the game. It's easy to level up in like an hour, but it's not easy to have the money for new equipment every level.
Quests:
Have a few unique quests open up at level 100 for some unique rewards like maybe 5 more spaces in your inventory or for maybe a cool weapon, or a few additional points to stats. Maybe you can revisit these quests at a higher challenge after you leveled to like 130 or 160.
Perks/Bonus:
Maybe when a player reaches level 100, they can acquire a special perk or bonus goody or something for working so hard. Maybe have like a small crown symbol on their head or by the name to show they are level 100+.
Feel free to add anything you think might be cool to have after obtaining level 100.