I have an idea for an RPG. All kinds of swords (scimitars, fleurets, sabers, broadswords, etc) are in one category (just Swords), so when you improve your skills with swords, it applies to all kinds of swords.

Two handed swords, however, would be in a different category, but that's another story.

What do you think about this idea? do you know any game that has done this before?

then what's the difference between these swords?

Sounds like Diablo2. D2 didn't differentiate between one handed and two handed swords, but it did differentiate between swords, maces, axes, etc. It's not a significant difference IMHO.

So RPGs often have stat++ for weapons/magic you are currently using. Positive feedback for sticking with same weapon/magic. Encourages specialisation.

Your idea is do stat++ for all weapons/magic regardless of which you currently equip.

Why not do stat-- for weapon/magic when used? Or alternatively, stat++ on all other weapons/magic except the current one, which stays constant. Would encourage variation rather than specialisation and could give interesting gameplay (don't use special sword; keep for special occasion like final boss).

The universal good answer is that it depends.

• Do you want switching weapons to be good or bad for the player?
• Is it a long term commitment (e.g. limited inventory slots forcing the player to leave old weapons behind) or a short term tactical choice (e.g. specialized weapons for specific uses, like maces against animated skeletons in D&D: all carried at all times, requisitioned from the armory before short missions, etc.)?
• Is it a common problem that should affect enemies too (e.g. preferred weapons abandoned for spares during combat because they get damaged), an uncommon opportunity (e.g. you find an enchanted exotic weapon) or a rare disaster (e.g. losing magical weapons that improve progressively and are far better than anything else)?
• Do you want to encourage weapon changes? When and why? Is the player character expected or forced to change weapons (long term) at some point in the game?
• Your idea is to give “free” skill points for weapons that aren't exactly like the one the player is using. Why do you think that different types of one-handed swords are a good choice for this privilege? You exclude two-handed swords and you don't mention smaller bladed weapons. Are you trying to shape specific “character build” choices?

What do you want to achieve with it? Is there a real choice to be made? In real life it's different fighting with a gladius than fighting with a longsword yet they can both be handled with one hand. But gameplay wise I would find it tedious if I have different skills for each and every weapon.