Here are a few options for some DLC in DJ Hero I would like to see make the cut: 

-Deadmau5
Somehow, somewhere he needs to be in. Tracks such as 'Faxing Berlin', 'Hi Friend' and 'Reward is Cheese' would make great backing tracks at the very least due to their heavy basslines. As for actual songs of his, I'd say put in Ghosts n' Stuff or the remix of Daft Punks 'Harder Better Faster Stronger'. He has more than enough songs to fill a DLC package so this should definitely happen.
 
-Chase and Status, High Contrast, Caspa, Rusko or Sub Focus
British fans of DnB and Dubstep will be more familiar to these artists as they primarily produce in the UK, however it would be nice to see a pure Drum and Bass DLC package for fans of it such as myself. Noisia and Scratch Perverts also make that list but thankfully they have already been graced by DJ Hero :)
 
-Armin Van Buuren
Mostly a trance producer but still able to pump out some sweet tunes, it would be nice for him to grace the decks with some of his well known tracks from A State of Trance.
 
-Calvin Harris
He's done plenty for the dance music scene over the last 3 or 4 years and it would be nice for him to have his own DLC package made from tracks from his new album and old. Along with collaborations with Dizzee Rascal there would be plenty to choose from.
 
Those are my ideas so far, feel free to add your own and why they should make it in. I know my choices dwell on the dance scene, but that is a personal favourite of mine and it is my wish to see more of this genre in DJ Hero. Any genre is welcome so long as it makes sense I guess!

the only problem with you list is  that these artists aren't necessarily known for mash-ups, which is what this game is based around.  I am supprised  Girl Talk  didn't make an appearance, he's pretty underground but is well known for his insane mashups.

Daft Punk's 'Alive 2007' album.
 
It's perfect for this kind of game.

I think Jamiroquai would be a perfect fit for DJ Hero. So many of their songs have been remix by various club/house DJs. Songs like "Canned Heat"(remember the Napolean Dynamite dance scene), "Supersonic", "Little L",  "Space Cowboy", and "Virtual Insanity" just to name of few.

Not that I ever think it'd get made, but I'd dig a 2P Shadow / Cut Chemist thing, in the style of Brainfreeze / Product placement.
 
Aside from that, Dubstep Hero sounds ace.

It's Activision. There won't be any DLC coming.

more rap and more crazy techno