I noticed in Space Dock (page 130-133) in the shield description section several shield modifications that could be integrated into a shield grid and thought the natural progression is to develop enhancement technology for any shield generator.
I won't add metaphasic, widespread implementation by 2380's and by 2400ad all starfleet vessels have regenerating/metaphasic shields., and as per Space Dock this mod doesn't increase SU's or power use.
Multiadaptive shield enhancement, this unit ties into the shield grid and modifies the shield to act as a Cloak 6 (+6 difficulty to detect vessel) while ever the shields are active. Obviously this modification would be inappropriate for most StarFleet ships as it violates the Treaty of Algeron, though after the destruction of Romulus in one time line the Treaty would be amended....
SU=sc x 3
Pwr= normal shield power cost + scx3
Beta-Tachyon pulse and anti proton enhancement (only available after 31st century and by this time all vessels would be using this enhancement [not canon but I reckon it would be the case], though very limited usage could be justified after 2378 and the return of Voyager) this powerful enhancement strengthens the shield grid by re-modulating it to emit a Beta-Tachyon pulse and then projecting anti-proton beams into the grid.
This increases the shields Protection and Threshold by a factor of TEN.
Power cost 30/rnd when the enhancer is active.
SU=sc x 10
This modification was shown to Voyager by the female Q, and allowed Voyager passage to the Continuum.
http://en.memory-alpha.org/wiki/Beta-tachyon
http://en.memory-alpha.org/wiki/The_...Grey_(episode)