The way I see it 8 slots is enough to have both mandatory slots and expressive slots. I dont see an issue with serration being on every build, it's that every crit build is the same. The issue is just that too many mods that buff damage exist.

You've got damage, damage + proc, damage + proc + status, crit, crit chance etc. There's just too much competition for utility mods to see use.

I would suggest you just trim the fat. Make the elemental mods just convert damage types. Make the damage + element mods status chance + reduced element type to distinguish the 2. Instead of just stacking damage and killing everything without a second thought that 15% buff to flesh actually means something. (Maybe buff that too).

Things like that are where I would start. Of course no one likes nerfs and many people dislike that level of management. It's a tough problem.