I'm working on a Resident Evil 1/2/3/0/Remake type of game. Currently I've done a big part of the inventory system (here's a link if you wanna see my inventory, pretty outdated, added a lot of features and made a lot of enhancements)

Now I'm thinking about how to approach the items system, If you've played any Resident Evil game or any of its likes you should be familiar with what I'm trying to achieve. Here's a very simple category I made for the items:

So you have different items, with different operations you could perform on them, there are usable items that you could use, like for example herbs and first aid kits that 'using' them would affect your health, keys to unlock doors, and equipable items that you could 'equip' like weapons. Also, you can 'combine' two items together to get new one, like for example mixing a green and red herb would give you a new type of herb, or combining a lighter with a paper, would give you a burnt paper, or ammo with a gun, would reload the gun or something. etc. You know the usual RE drill.

Not all items are 'transformable', in that, for example: lighter + paper = burnt paper (it's the paper that 'transforms' to burnt paper and not the lighter, the lighter is not transformable it will remain as it is) green herb + red herb = newHerb1 (both herbs will vanish and transform to this new type of herb) ammo + gun = reload gun (ammo state will remain as it is, it won't change but it will just decrease, nothing will happen to the gun it just gets reloaded)

Also a key note to remember is that you can't just combine items randomly, each item has a 'mating' item(s).

So to sum up, different items, and different operations on them. The question is, how to approach this, design-wise?

I've been learning about interfaces, but it just doesn't quite get into my head, I mean, why not just use classes with the good old inheritance? I know the technical details of interfaces and that the cool thing about them is that they don't require an inheritance chain, but I just can't see how to use them properly, that is, if they were the right thing to use here.

So should I go with just classes and inheritance? just like in the tree I showed you? or should I think more about how to use interfaces? (IUsable, IEquipable, ITransformable) - why not just use classes UsableItem, Equipable item, TransformableItem?

I want something that won't give me headaches in the long run, something resilient/flexible to future changes. I'm OK using classes, but I smell something better here.

The way I'm thinking is to possibly use both inheritance and interfaces, so that you have a branch like this: item -> equipable -> weapon. but then again, the weapon has methods like 'reload' 'examine' 'equip' some of them 'combine' so I'm thinking to make weapon implement ICombinable?... not all items get used the same way, using herbs will increase your health, using a key will open a door, so IUsable maybe?

Should I use a big database (XML for example) for all the items, items names, mates, nRowsReq, nColsReq, etc?

Thanks so much for your answers in advanced, note that demo 3 is coming after I'm done with items :D