To get it right out of the way, this suggestion is targeted at the abuse of "Shield-Cancelling".
My suggestions do not eliminate Shield-Cancelling. They focus mainly on those people who have become so proficient with it that they essential become an unbalance in play.

First: In Lockdown, make letting go of the shield within 2/3 of a second deplete a small portion of the shield. Possibly less shield consumed on Guardians, but I'm not sure.

Second: In Lockdown, make it so that if you attack, then shield, your next attack will instead be the next attack in the cycle. This, to, would be a relatively short effect. Probably around 3/4 of a second or a full second.

Basically, this lets them keep using the technique, but with slight restriction and without harming the players ability to hold their shield at length.

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Just tossing that out there. I know these things tend to quickly split player-bases on what should be done. There's Twinks in WoW, Metaknight in Brawl, Fighters in Dungeon Fighter, and don't even mention Gunz to anybody~
Apart from Gunz, the abuses never really bug me, but the debates and name calling get a little tired after a while....