MArcomage

I am not sure if the game needs any new keywords.
I am also not sure for what this keyword should do :P

But at least conceptually, it will give the game some more depth. I mean every fantasy game needs artifacts, right? :)

Cards that will have the artifact keyword will strictly be objects (a sword, a shield, a cloak etc).

I'm thinking that maybe it would be something similar to Legend: not so many cards, maybe only uncommons and rares.

Existing cards that could go into this are: Scepter of summoning, Soul cleaver, Globe of Myr, Anomaly Sphere etc.

I've given some thought in the past to a keyword such as this. I was thinking Weapon, and some of the "equipable" cards you described would apply. (At the time I was thinking of this, I believe Renewal may have been named to be a weapon as well, though I may just be confused given its picture.) With such a keyword, other cards could be written to do more damage if there is a weapon on hand, or some spells could target such objects for their destruction. I didn't go so far as to figure out an effect the keyword could have on its own, however.

It's a very abstract idea on my mind too. :)
I wouldn't restrict it to just weapons though. It could be a ring maybe.

I was thinking something similar to Enchantments like in MTG. I'm not sure that this is implementable though as they could potentially modify the effects of every card played.