I would indeed say that having weapons allowed to be upgraded by crystals and adding crystal-fusion to upgrade said crystals would be an awesome way to implement alrdy existing aspects of the game more in to the everyday gameplay which would be awesome imo
Having the reroll system seems like a bad choice since it seems like more of a broad daylight-robbery of unlucky people, seeing as those who are onlucky might have to spend tens of thousands of plats to get the perfect weapons while others only spend 500 plat getting all their gear perfect..
Instead i think crystal fusion and crafting should be bought with plat only, as to hopefully make plat a little more wanted and needed currency, it seems insane to me as it is right now with plats being actually less worth than creds..