You could use one of the afforenmentioned “midi control surface designer programs” (thats what they are really) TouchOSC or Lemur (I personally recommend Lemur) and not only control Traktor effects with it but actually (due to new ways of interacting with midi controls) go far beyond the typical sound they could-would produce.
Imagine X,Y pads, modulating faders-knobs which interact with each other, custom made touchable LFOs and all sorts of wicked stuff.
TO further understand check out Lemur’s user library to see what is currently possible to be created (again this is ONLY a speciment of what its users have created, your imagination is the limit here)
http://liine.net/en/community/user-library/
or these Traktor specific ones
http://liine.net/en/community/user-library/tag/16/
Or (another small example), how about controlling Traktor effects and other parameters by moving your iPad in the air
http://liine.net/forum/viewtopic.php?f=34&t=2913
😉
P.S. edit: NO Im not in any way affiliated with Liine-Lemur lol. I have been using midi controllers since 2003 and when I got Lemur it was the first time I ever felt “free” to realize my dream controller, thats all – and it was the first time I actually felt limited by my own finger-skills and imagination rather than by the surface in front of me! 😉