Small shields: Would have the ability to half any incoming damage that is done to it. Has weak regeneration rate.
Medium shields: Would have high regeneration ability that enable it to refill it's shield bar very quickly. But a strong attack would break it easily in one or two hits.
Large shields: Would have the ability to hulk down any attack thrown at it and not get knock back. Would also negate that attack so that it does not inflict any damage on nearby allies. Edit: Large shields have average shield regeneration.
These suggestions for shields is to give shields more variety then just defense. This would give them types and would give them more use when thinking tactics. I feel these suggestion would improve the play style because shields would not merely mean defense, they will mean a certain play style that players can cater too even if not labeled class, it would kind of define class types in spiral knights.
Small shields would fall under swiftstrike buckler, proto shield, iron buckler, force buckler, Darkfang Shield and green ward.
Medium shields would fall under Blazebreak Shield, Blizzbreaker Shield, Scarlet Shield, Surge Breaker Shield, Aegis, Barbarous Thorn Shield, Celestial Shield, Crest of Almire, Dragon Scale Shield, Dread Skelly Shield, Grey Owlite Shield, Heater Shield, Power Mitt, Royal Jelly Shield, Exalted Honor Guard, The Bitter End, and The Final Gasp.
Large shields would fall under Ancient Plate Shield, Gorgomega, Grand Tortoise, Ironmight Plate Shield, Omega Shell, Omegaward, Savage Tortoise, Volcanic Plate Shield, and Node Slime Wall.
Now some suggestions for shields with special abilities:
Force Buckler: Has the ability to create a vial stun effect upon the shield breaking. This stun last as long as a tier 3 vial. But this stun also boost the monsters attack during the stun duration.
Green Ward: shield bar regenerates faster upon the knight being inflected with the poison effect.
Blazebreak Shield: Ricochet any fire damage that is thrown at it at another direction which can damage nearby monsters if hit by it. Very similar to the alchemer gun. This does not however negate that damage.
Blizzbreaker Shield: Ricochet any ice damage that is thrown at it at another direction which can damage nearby monsters if hit by it. Very similar to the alchemer gun. This does not however negate that damage.
Surge Breaker Shield: Ricochet any shock damage that is thrown at it at another direction which can damage nearby monsters if hit by it. Very similar to the alchemer gun. This does not however negate that damage.
Scarlet Shield: Fake death which instantly gives the knight one more pip of health after being killed. Can only use once per level. After the shield has given the knight that health, it can no longer help boost the knights health for the remainder of that level.
Darkfang Shield: 25% of all fire and shock damage that is inflicted upon it is taken as shield regeneration which enhances its regeneration for a short period of time.
Aegis: Piercing and normal attacks boost the shield's shield bash attack damage value. Can only boost it by 50% if the shield is low on energy.
Ancient Plate Shield: All normal damage increases the knights chance of completely negating an attack and receiving no damage reduction for that one attack. The chance bar is carried over as long as the knight is in the clockworks and can be carried over to other levels. The bar is reset every time the knight get's that chance.
Barbarous Thorn Shield: "Black gaze" When ever the knight block an attack from a monster, that monster's defense is reduced by 20 - 30 % for 5 to 10 seconds.
Celestial Shield: Upon receiving a large attack, the shield decreases that monster's attack for 3 seconds, and increases the users attack by 50% for 2 seconds.
Crest of Almire: Upon the knight activating his shield, all undead monsters gain a small agro percentage towards the knight.
Dragon Scale Shield: For every time the knight using the shield is inflicted with poison or fire or both statuses, the shields defense in increased by 50% for fire and 50% defense increase for poison. If the knight is inflicted with both statuses, The shields defense values are increased by 100% during the duration of the status infliction's. When the inflections are gone, the shields goes back to normal.
Dread Skelly Shield: All undead monsters regard the player as one of their own after the shield is broken. After the shield fully recharges, they will notice the player as an enemy again.
Grey Owlite Shield: While in any candle themed dungeon, The shield can act as a momentary lite candle upon shield bash. This effect last 10 seconds and has a cool down of 5 seconds.
Gorgomega, Grand Tortoise, Omegaward, and Savage Tortoise: All tortodrone theme shields gain 25% defense increase upon their shield bar going under half way and 50% increase on their shield closing to break.
Heater Shield: The knight can escape being frozen upon using shield bash.
Power Mitt: Upon shield bash, the knight can walk through any fire them hazard without being burn for 2 - 4 seconds.
Ironmight Plate Shield: Any large knock back base attack is repelled and thrown back upon the monster knocking it back equal the amount.
Omega Shell: Upon shield bash, the knight is invulnerable from damage during the duration of the shield bash.
Royal Jelly Shield: All slime/jelly monster base attack is absorbed as if a Royal Mini/mini jelly cube is attacking.
Volcanic Plate Shield: While the knight is on fire, all damage inflicted on the shield is negated.
Exalted Honor Guard: While on a mission, the knight will get a free companion that is invulnerable to damage. This companion looks like a scouter and aggregates agro from all turret monsters towards it. It will disappear upon the shield breaking, and comes back when the shield fully heals. This companion does not attack and just follows the knight around with a robo knight shield to block incoming attacks. The knight can also command the companion to pick up minerals, or keys, and can even serve as an extra inventory slot for the knight to carry things. Though all items on it will permanently disappear upon it disappearing when the shield breaks.
The Bitter End: Upon the knight carrying this shield, all npc knights will salute when approaching near that knight. Also all npc knights/robo knights and turrets and will get a boost in defense while inside the clockworks.
The Final Gasp: Upon the knight carrying this shield, all npc knights/robo knight, air strike and ally turrets will get a boost in attack while in clockworks. And all npc knights will bow emote when near the knight.
Node Slime Wall: Shield bash ability geyser will take all damage inflicted upon slim monster during the shield bash to heal the knight. Also any jelly base armor the knight is wearing will get it's defense boosted by 50%.
I feel i might have been lazy in my creation of these shield abilities, but give them a thorough discussion to see if they fit. Hope you got some replacement ideas though if they do not fit. Thanks for reading :) and i hope to read productive criticism.
But I like especially where the Medium shields do>I presume you mean to say that large ones have average regen rate.
As for the abilities,they're basic but decent as you imply,but I especially like grey owlite's as it gives the miniature dilemma of whether to get a shadow shield for defence or this one for level practicality.
Another thing I suggest is shields that INSTEAD affect one's dash/bash.Like a category that's faster w/ less cooldown but covers less distance,going a little astray yet along w/ your formula at the same time.