Back to topic. Let's exclude GURPS from the get go ( since to all the post in the thread, GURPS would be one of the answers by default).
I was thinking about running military themed campaign, centered around group of WW1 stormtroopers. Setting is WW1 but conflict didn't really ended, so much as it stalled, with trenches, acres of no mans land with landmines, occasional offensives and counterofensives. Let's just say that someone figured out that status quo forever war is good for global elites to retain their power. So the war kind of goes on ad infinitum. It would be mostly be mission based - as in- one session, one mission. Trench raids, railroad sabotages, outpost raids, outpost defense, that sort of things. So, system that has solid firearms combat mechanics, maybe some decent rules for mass combat, but easy enough to make starting character in 30 min.