I love riddles.
They've been an integral part of nearly every pnp campaign I've played in. They show up all over the place in the old modules (possibly the new stuff too).
DDO has traps and puzzles and secret doors, which are also pnp staples. Why no riddles?
If the answer is "because people would memorize the answers", well, yes, they would. But people already memorize the layout of puzzles and the location of trap boxes and secret doors.
Riddles, please!