I like Rizos idea. with time limited things on epics from events, from events etc.
S grade. I think it is to early to make S grade yet? Seems like people dont even have A grade +6, like most of people, and [A] grade weapons +10. Just my humble opinion, it is to early, with new update, it is easier to get gemstone A which was main problem to craft A grade weapons, soon people will OE [A] grade weapons, which is cool.
RB's. Fck those rb farmers. IT is hard to balance that, from other side as u said Rizos, making limited time items will stop farmers to farm, but from other side, new people want to kill RB not just for exp but items as well, it is free 1kk-20kk depends on rb lvl and drop.
What I would change instead of limited time on items is, if u get item, u can sell that item ONLY to your party member who was with you when you killed boss. And then BIND that item to you and your account. So it wont be useless entirely.
What do I mean, it is like in WoW. For example. Boss dropped SLS, you picked, but u dont need it for example. YOu have 2h to sell that SLS to your party member. Your party member buys SLS from you. He is SK, he uses that SLS for farm, etc. He cant sell that item to anyone outside party, 2h ends and he cant even sell to his party member. He cant make crystalls out of it. But at lvl 52 he can use B grade weapoin, so basically SLS becomes useless. So what do I suggest is, at least make that SLS bind to account. You can send that SLS to your Alt, or something, like only from same IP to same IP. Bind to account item. I think it will be a lil better than just limited 50h. But idk, just an idea. Maybe it is hard to make or even impossible.
p.s. Fck u rb farmers.