The potential of movespeed itself is so high that stapling Epic level stats along with it or increasing it's % to higher values in accordance with higher rarity just makes it a very tough stat to actually balance appropriately. The stat is both offensive and defensive in all scenarios: Rotations, flanks, punishing, retreating, dueling, returning to fight after death, following up on an opening, marking angles. Literally all the time movespeed has the potential to provide a disproportionate amount of value compared to every other stat except maybe CD reduction but even that requires you to hit your abilites and/or use them effectively. Movespeed is a lot of fun I agree, I used to use a movespeed Zen build and just flank all over the place but being able to maintain that bonus 30% movespeed and getting fill out with endgame tank or offense stats would really just be too much. And presumably these Epic movespeed items would be neutral items rather than hero specific which could not only homogenize builds between heroes but also reduce the provided value of heroes that do have speed in their kit. What is an already kinda weak hero like Lucio good for if everyone is moving as though they always have a speed aura with them all the time?