Not entirely sure how you separate katanas, longswords, sabers, etc into distinct branches of a "Swords" skill tree that actually make each one feel different, and not just be +Parry, +Crit Damage, +Attack Speed, -Stamina Cost, etc. And then balance that against separate skills for however you want to group axes, maces, hammers, spears, unarmed, etc. And deciding between what should scale with our skill, and what should scale with the upgrades/modifications we can make to the weapons themselves via crafting.
I think it would be more prudent to talk about the ways they can expand melee combat, differentiate different styles of combat, and make different strategies viable in different situations.
- Skill trees should focus on general weapon categories, and include branches for different playstyles like sword-and-board, sword-and-spell, dual-wielding, one-handed dueling, parrying. The stats of the weapons themselves should differentiate between the various weapons (crafting upgrades should reflect that - greater power attack damage for blunt weapons, bleed damage for axes, armor-piercing, better crits, limb damage, etc).
- Taking after Fallout, melee weapons only get timed parries - wielding a one-handed weapon with nothing in the off-hand improves this. Shields can still be held up indefinitely, and blocking an attack reduces stamina instead of health.
- Probably more important than anything in the character system or different weapons, different enemies need to call for different strategies. Give skeletons, dwarven automatons, and other fleshless creatures a resistance to bladed damage and a weakness to blunt damage. Give ethereal opponents a weakness to magicka and a strong resistance to conventional damage. Add enemies with armor plating in specific areas, and enemies that can be dismembered or crippled to turn into an easier opponent. Enemies with absurdly high armor rating and absurdly low HP, weak and fast enemies that swarm you, weaker enemies that buff stronger enemies or throw debuffs at you, weak conjurers that summon tough Daedra. That'll improve combat more than any skill tree or a million different weapons.