So for the longest time, people've been wanting a new weapon skill line, if not multiple of them. From the arguments of Crossbows and Spears, to wands and spell books and MANY others, there are valid reasons for new skill lines to be added. However I believe there is a slight problem with the weapon skill lines that would need to be addressed before adding new skill lines would make sense, lest every skill line be a two-handed or niche skill line.
Here's the proposal: cut sword and shield in half into two skill lines. Now hear me out. Keep all the skills that exist in "Sword and Shield," and put them in the new skill line that would be called "Shield," then make a brand new skill line called "Sword" (or one-handed, I suppose to mirror two handed). This would create a new skill line, but set the scene for new skill lines down the road and give access to something else for everyone that uses those skill lines.
Here's how this works: you can use the sword skill line by itself, but you'd be kinda gimped when it comes to your damage since you're only getting half out-put by wielding one sword. Same with shield but worse, since shield doesn't let you attack with it, not really anyway. But by wielding the second option, you'd gain access to the second skill line. So, with what's currently there, you get the sword skill line AS WELL as the sword and shield skill line if you wield a sword and shield, and you get access to the sword skill line as well as the dual wield skill line if you wield two swords.
This would open up the option to add more one-handed weapon skill lines. A one-handed crossbow skill line for range, a wand skill line for magic, short spears, thrown weapons, the options open up with this, since you'd be able to customize them by dual-wielding with the other weapon skill lines. Then, ZoS would only have to make alternate versions of the existing skills in skill lines, changing slightly based on the other weapon in your hand. Magic sword and board? That's wand+shield. Ranged Dual Wield? Two crossbows or a crossbow and sword. The combinations are vast here, and it would open up diversity in the game even further without much of a huge change, since it's only additive.