Yes!!! This seems like a very cool idea. Some of the cards in Spells of Genesis look very cool and I think it'd be even cooler to see them in another game. I'm sure some people who are into both games might even try to make characters with similar names to cards that look like the cards character! One question though to do this requires Bit crystals to be on board with your idea doesn't it?
Would like to add on top of the question asked by GamingOn how does this affect the development? Will developing these cosmetics be costly financially or timewise? If it's costing to develop these custom cosmetics where is the spending budget coming from?
Besides that I think it is not a bad idea but not one I'm particularly interested in. If I end up playing Project Rebirth, likely I wouldn't want to go through the hassle of getting the SOG asset but just buy it off another player in game because some of them look damn awesome but what can I say? I'm lazy.