All Mordekaiser needs is CC or mobility (more than his W provides).
However, our current iteration of Mordekaiser would likely be overpowered with the addition of mobility or CC to his kit.
Riot doesn't want to give him major changes right now, but major changes will be necessary for him to be truly valuable to a team, rather than an immobile deadweight that either does no damage or dies immediately upon reaching his opponents.
Tanky Morde applies moderate sustained DPS to his opponents, but he can't force them to stop hitting his carries (or sit in range of his abilities).
Glass-cannon Morde is usually killed before he can get in range, and he has no way to keep himself alive in a fight beyond spamming his skills and hoping he never becomes the target of any CC. Morde can't keep his shield up when his abilities are on cooldown... Or when he's hit by CC.