Here is another idea. A waterproof Ortleib style suit bag. Panniers are not an optimum size for a suit, so for someone who rides to work everyday, it would be handy to have a good way to carry your suit.
1. The suit backpack/messenger
Suit bags are generally pretty lightweight, so this seems feasible.
Features: slightly larger than a messenger bag, unzips and folds out for hanging like regular suit bag; perhaps slightly harder walls, so that the suit did not get crushed (but weight main priority); waterproof - obviously a vital necessity; clips for LEDs and some other pockets and general messenger bag features.
2. The pannier style suit bag
This could be a little more substantial perhaps. This idea stemmed from my use of a regular (somewhat waterproof) suit bag sitting on top of the bikerack, with the coat hanger hooked onto the seatpost, and the plastic of the suit bag clipped onto the panniers to stop it from moving. It is a good way to carry the suit, but a commercial version could be so much better - properly waterproof, easier attachment, clips for LED lights -I'm thinking something like one of those great big folders that artists use to carry their work around - but when you unzip, it is a folding suit bag. It could be lovely.