Are you looking for a KNEX gun/gun-related IDEA???
I was looking at the gatling/machine guns on this site and I thought: hey! if you want to build something cool and incredibly useful, you should try and build something that helps to reload these guns, or makes more ammunition for them!
it doesn't have to be automated at first, just a crank that takes a rod and/or rubber band and reloads a spent ammo belt from your favorite automtic knex weapon would be a good start!!
Happy hardware hacking!