I think shields that do not give good protection but increases the players dynamic would be pretty interesting.
The general idea is to get a higher asymmetry in the players ability and weaknesses in a way that with good tactics a party is stronger if they compensate each others weaknesses.

So my suggestion for a 4 and 5 star version of the Swiftstrike is:

4* version of Swiftstrike:
Defense Power:
same defense damage as 3* Swiftstrike.
LESS defense health as 3* Swiftstrike.

Abilities
Attack Speed Increased: High
Shield Pushing power increased. (the ability of push enemies away if they are inside your shields protection sphere)

5* version of Swiftstrike:
Defense Power:
same defense damage as 3* Swiftstrike.
LESS defense health as 4* Swiftstrike.

Abilities
Attack Speed Increased: High
Charge time Reduction: Medium or Walk speed increased: low.
Shield Pushing power increased.