There are a few things I want to see in DLC (if we get DLC):

Mainly from this video (wipeout XL; Talon's Reach) I would like to have the following in HD:
http://www.youtube.com/watch?v=7_VbjOxsA0g

1. the interesting textures for the track, and the black background.
2. the shortcut. I dont know if HD has big shortcuts like that.
3. the electronic voice as an option in settings. Don't get me wrong, I like the voice in HD.
4. the smoothness and speed of that track. maybe a track built for zen speed with at most 1 BR per lap, or no BR at all. Scraping the walls might get annoying though.
5. red bull ads. at least 1.

Wipeout HD is my first wipeout game, and I've been watching tons of wipeout videos since I started; the XL videos look the most interesting. I just picked up XL to try it out.

other thoughts:
Sorry if I talk about F-Zero a lot here, but the last racing game i got into was the original fzero on snes.
6. F-Zero on the SNES would give you a little bump forward when your engines powered back on; it was something I really got used to. So far I haven't seen this in wipeout, but maybe I missed something with one of the ships. Hopefully they'll add a new ship that gives a bump when igniting the engines, but I abused this too much.
7. Something similar to a blue shell to take out the leader. just dont give it to the computer AI. This isnt a well thought-out idea.
8. avatars
9. I was sitting at the multiplayer screen waiting for the host to press the "start" button for like 5 min, and he never would. It would've been nice to somehow know what his intentions were, or if he was afk. Someone mentioned the idea of doing TTs until the MP game starts; that would be useful. I got bored and played single player; which probably made him wait longer.
10. After a race there's no way to check to see how far you were from a medal. You have to return to the grid to see the gold/silver/bronze time requirements. It would be nice to have that information from within the event.
11. the ability to watch/save replays, and take screenshots during replay playback
12. split-screen online (this should probably be at the top of the list)
14. After your car explodes you can let off the gas and then re-ignite the after-burner. pretty funny, but I wouldnt call it a bug. Maybe it should fire confetti out the back?