I agree with Mal on the W, I feel that it creates a window of power much like crowd control and mobility does on other champions, and I think his W is what Riot sees as makes him have duo-lane potential. I just think that there could be more to the W than just bonus experience, which doesn't fit him thematically and honestly doesn't do much except promote the 'snowball harder' issue that has always plagued Morde.
I think that Mordekaiser could afford to have his relatively weak base stats if he got more of those 'moments of opportunity' to make up for it, I've suggested several times on the forums that the bonus experience passive could be replaced, perhaps he could get scaling armor and magic resist in some form when using his W.
Hm, maybe said armor and magic resist could scale further from his own percent missing health? Thematically Mordekaiser gets power from pain, so he could deal more pain if he is suffering more, and he thrives on pain so it makes sense that he could survive longer in fights if he is suffering that much more.
Plus, using his own percent missing health, rather than using bonus health or AP or his innate resists would prevent the scaling from indirectly buffing snowballing on Mordekaiser.
My other gripe is with dragon ghost: I love it, I want to keep it, but I feel it costs Mordekaiser nothing to get it... mostly since all it does require is Morde damage the dragon before his team takes it. I feel that making Morde have to cast his ult on the dragon would make it not so easy for Mordekaiser to get that ghost, and would inhibit snowballing on Mordekaiser since right now he could take a marksman ghost, take tower, then go do dragon and get ANOTHER ghost, and likely have his ult up by the time the dragon ghost dies.
That whole thing was one of two reasons why Mordekaiser was pick or ban during the World Championship (the other being 100% bonus experience).
With that change, getting the dragon is an option, but getting the dragon ghost is a decision, a decision to save it for the dragon and thus not have the chance to use it in teamfight, or to use it in said teamfight and not have it immediately available if the dragon is rushed after a successful teamfight.
That would open up a bit more for Mordekaiser's power budget for other things.