June 2nd Goat-ertainment - a puzzle!

My goat-goal this month is to post something interesting every day of my goat-dom.

Today I have a puzzle for you! Here are the rules:

1. I don’t care if you break the rules.
2. Send me guesses via whisper, so other people get a chance to figure it out.
3. There is no prize.
4. I don’t think Google will help you much, but you’ll have more fun if you don’t use Google.
5. Did I mention there’s no prize? Well, there’s no prize.
6. It may take me a little while to reply to your whisper, so don’t freak out.
7. I might post hints in the comments.
8. Feel free to discuss in the comments, but try to avoid spoilers if you can. If you definitely don’t want spoilers, don’t look at the comments.

Now, here’s the puzzle. These letters are in chronological order:
PDR
SLP
JRE
JIL
JFK
JDQ
???

What letters come next in the sequence?