For base maintenance, the use of GPUs could be minimised by having a centralised power generator and air generator placed off the hangar's premises and having the relevant power cables and air ducts placed at strategic locations around the hangar's floors. This upgrade will not be feasible on an existing hangar as the renovations will disrupt the hangar's operations greatly.
Another idea which has been floating on my mind is to line the hangar's interior walls and ceiling with noise absorbing material similiar to a recording studio. The majority of the noise emanating from the hangar is actually noise which is amplified by the hangar's interior reverbrating around the hangar. To be fair, I don't think any MROs have this in their hangars as the related maintenance cost of maintaining the noise absorbing material, not to mention the acquisition costs, would far outweigh the benefits it brings along.
And if your MRO allows the operation of APUs while the a/c is inside the hangar. Ban it immediately.