This might be crazy.. but what if Morde got a Movement Speed passive
New passive on his Q With each strike of Q on his first and second strike Morde gains 5% and then 10% movement speed bonus (total 15%). While Q is being stacked, after the first strike, Morde can not be slowed (Is not immune to CC but can not be slowed, same as Yi ult but not as much movement speed bonus).
Numbers may be off but if Rylais gets a heavy nerf this may be an option?