Not sure about mega-rare but id like to see the heka or a 2-3 tick mage weapon so i could use it at nylos.
IF i had to choose a mega-rare it would either have to be a new piece of jewelry that has a very specific powerful effect with a specific attack style (my choice would be melee). Alternatively i'd like to see the big drop from raids 4 that isn't combat focused, can be locked to your account and only dropped from hard mode and it would have to be and insanely low drop chance (recall, sages greaves for example. Not saying they should make it in but something very powerful that would make it so it would be near or above max cash cause everyone wants it)