Cape of flying as a quest reward or a wish-only item sounds good, I absolutely agree with you about giving the players more choices. Unfortunately, IVAN suffers the bane of many old roguelikes, having a set of "ascension gear", the best items for their slots - boots of agility, belt of levitation, cloak of fire resistance. (That's not to say I'm never useing other items in those slots, those items are just very, very good even compared to all other equipment available.) It would be nice to mix things up a bit with more items also worthy to be in your end-game equipment. BTW, ommel hair or expensive fabric (also red) sounds great for the cape of flying.
Cape of quickness giving Agi is also an idea I like. We should have more options for cloaks than simply "pick one resistance". Hm, I could try doing a cloak of protection - made of metal, but has no inflexibility penalty, thus giving great AV in exchange for no special powers.
For other multi-slot items, maybe a full plate mail, giving you huge AV, but using up your body, head, gloves and boots slots. Hm, what would happen if one of your hands got chopped off while you were wearing the full plate mail, loosing one of the slots it is equipped in?
And if you want to try out your capes, just put them under cloak base item in item.dat, defining them in define.dat after the current cloaks, and that should be enough.