I'm still working on this, but I will tell you my thought process:
I'm treating the decker as the Shadowrun analog to the thief. He sneaks into places he isn't supposed to be without being seen, he can deliver nasty sneak attacks, he can open doors and other "locked" things, he can filch valuable data in an underhanded fashion, he can make devastating surprise attacks, and he must be wary to detect and disable traps left to cause him harm.
I'm also contemplating two treatments of the Matrix,: analog and modern.
Analog Matrix is that of Shadowrun 3rd edition and older, as well as a lot of 80's and 90's cyberpunk fiction: the Matrix is a virtual world that users can interface their minds with directly via datajack.
Modern Matrix is an omnipresent web of wireless networks, most with "agmented reality" holographic popups visible to those with the right gear. Everyone has their own little "Personal Area Network" comprised of all their personal electronics, as well as any gear and cyberware with wireless capability.
My goal is to leave the decker running in real time with the rest of the crew, and to create a simple combat system that is similar to the regular combat system, as well as a system for resolving common Matrix tasks, which I am going to keep as analogous to thief skills/thieving tasks as possible.
At this point, I don't think I'm going to do technomancers....but I might change my mind.