My suggestion is to give the Scout something that you would actually use to scout
with. I've always thought it a little odd that a Scout
runs around carrying a hefty close-range weapon. He's supposed to be ahead of his team spotting targets, not killing them in 1 hit.
Hence as an alternative to the scatter gun, give him a laser pointer-type weapon
(I suck at names) which is invisible to enemies (except for a dot, like the sniper rifle but smaller), and when held on a target for a second or two 'lights them up' for the scout's team. After my recent L4D binge, I'm thinking give them an outline that your teammates can see through walls, and maybe show their health (depends on balance). It can also mark up engy buildings, with the same effect (although it'd definitely show health). It may not sound like much, but I'd be interested in seeing how knowing where enemies are would affect gameplay.
The secondary fire of this little laser could be a more intense laser that doesn't mark enemies, but can effectively blind enemies. This could be used to shut down snipers/demomen/heavies (key defensive targets) temporarily to let your team advance and to let you sneak around better to get more marks. You could probably also use it for a pretty epic rush - if you turned a corner to find a soldier or something in front of you, you could blind him and sprint past. If nothing else, you could more easily get behind the enemy, even in linear maps, and mess with them, even though it doesn't really do damage. You could get like half a point for a mark, and assists for kills on marked targets.
That's it for now - secondary and melee weapons are harder