You can’t. Everyone got their 99s easy already. No point now. You’d have to do another level to even the player field.
Or make it hard, get lvl 99 on a new difficulty and maybe make something attached to it only the newest 99 can get.
Maybe be able to change the character glow to purple or red
Make the lvl 100 Char +100% Magic Find Gained forever & unlocks 1 x Stash Tab & 1x Char slot online

no but what i can do is allow you to move your exp bar to keep increasing in number of exp gained
More exp to reach lvl 100 make it twice as hard than lvl 99