The most interesting use of this weapon, I think, is flying it first, then using its ability on itself. It's a Dive that doesn't reset at the end of the turn, giving you something with exponential growth - something we've only seen ingame in limited form (think sky blitz + dive).
Based on how Crusader's endow works, this card shouldn't be able to use endow/attune itself (or any weapon). Crusaders can still endow weapons on the field despite the fact that they've been flown but they cannot use endow on other creatures. I imagine this would work the opposite way.
As for endowing passive abilities, that allows you to get:
Airborne - Bypass Wings.
Chimera - Similar to playing 2 Chimeras if flown, bad idea.
Devourer - Good passive to steal. Doesn't add any attack but it is free if cost is based off attack.
Mummy - Mummy rewinding effect.
Obsession - Wouldn't carry over to cards in hand.
Poisonous - Only works when flown and eaten.
Ranged - Same as Airborne.
Swarm - Only affects HP when flown.
Undead - Skeleton rewinding effect.
Voodoo - Possible the most interesting effect. Does your weapon count as "you" in the way that if you are attacked with Ice lance your weapon can be frozen. If this were the case Voodoo would be overpowered because every point of damage you take (that doesn't kill you) is also suffered by your opponent. If not, then it only affects when flown.
In most cases it only affects the weapon when flown. If the ability endowing is the main attraction of the card then I think passive abilities are not the way to go, however if the +attack increase from endowing is what's desired, passives solve a lot of problems.