Fast is a must.
What I really like about Human Smoke in this MK is that he is the one MK ninja who actually feels like a ninja. All the other ninjas don't feel like quick, stealthy assassins. With Smoke's fighting style and his moves, he feels like he is the most legitimate ninjutsu ninja of the MK ninjas. I think they should do that with Robo Smoke as well. Make him fast and stealthy, maybe even give him a short sword or a small, fast weapon. I think he should have quick, low impact punches and kicks and be contrasted with Human Smoke by being an offensive ground fighter instead of the arial, juggle based defensive fighter that Human Smoke is.
But to add to Robo Smoke's quick "ninja"-esque fighting, I think he should have his hand constantly with only two fingers out (much like his current fatality) and the other arm towards the back of his hips, ready to reach for a weapon. It's also notable that Smoke's two explosive fatalities of UMK3 are some of the most memorable of the whole series and perhaps explosive moves or fatalities could be incorporated in memory of those.