That does sound like an interesting idea. Unfortunately, I don't think I'd be able to help you too much on this due it revolving around NPC's (which I've never looked into in terms of code) and using GUI (which is the one area of Crea modding I refuse to touch, due to it being obstinately difficult to code.

For the Dark Mage, the way you'd want to code it is probably a list for each boss of their drops (you should be able to extract this from the bosses themselves in their Tuning data). Then associate the remna type (though I'd encourage you to think up something other than using "Remna" for sake of avoiding confusion as it would do a different thing) with the boss that it's taken from. For adding stats to the item, I think that the stats are just an array on the item, so you should just be able to add the stat to that array and it should work.

It would definitely be a cool mod though! (Something else that might be cool would be the ability to turn multiple of specific refined drops or such into whatever you choose to call the objects you can use to improve items).

If you have any questions directly, ask and I'll try to help since I know my way around the code base a bit!