Shadow War is one of those examples of where they are an inch away from greatness.
Shadow war is so similar to old Necromunda that I can run an old Necromunda gang and Shadow War kill team in the same campaign.
If GW just came out with a line of autonomous warbands like in Inquisimunda they would have a freewheeling Galaxy wide extravaganza. They could even keep the old Kill Team profiles as military expeditionary forces that are part of a command hierarchy and have supply support.
The only thing the whole 2nd edition campaign system is lacking is variable income systems for some of the weirder factions (Orks, Eldar. Arbites). They also need a comprehensive rare trade chart for all crazy alien equipment that some slimy Rogue Trader might have for sale.
I could see galaxy wide skirmish working if the campaign had a more customizable and open ended flow. Feral Orks and Grots rising from the muck and building muskets and crossbows until they get enough scrap to make shootas. Eldar scouts on quest for lost relics and web way networks.
These are the archetypes that would fit well in a skirmish campaign. It would be really easy to isolate it in a generic hive world setting and let dozens of people mix it up with a handful of randomly generated scenarios.
Its an instant gateway into the 40K franchise.