1) Make a super rare "riddle" npc who appears in town next to the portals. He never appears at the beginning of the game, but randomly appears there while playing.
He is obvious, unique garb, and shouts something like "Pick the correct answer, to claim your prize" with a little caption about his head. He asks stupid insultingly easy riddles, randomly from a list that changes week to week. You get four answer choices, and if you pick correctly, you get a million gold, perhaps less for low levels.
The riddler is phased, and appears only to one individual at a time...never to everyone in a group. When teleporting back to town, you will see him, and trigger a 2 minute timer to interact with him and answer the riddle. (the timer targets afk botting).
Players who trigger the timer but fail to interact (especially consecutive failures) are flagged for immediate review. Players with a track record of answering the question are then prioritized for review in order of their inability to answer the riddles.
To make it fun for players who have good track records, he'll occasionally ask tougher questions about D3 lore, with bigger prizes than the million gold. E.g. maybe a few legendaries that only the riddler can drop, which will strongly encourage players to take answering the riddler seriously.
This is a terrible idea.
Why is this a terrible idea?
I am in all support of ideas that does a simple check to see if the player is actually a human, and rewarding human players with gold/loot, finding and identifying botters and promptly (weekly ban waves please?)