@SingingThroughTheStorm:
I think Haki and flying sword attacks were after thoughts.
No, they weren't. Haki has been planned since the beginning. Maybe some details came later, but it was there.
Arlong traveled into the Grand line and fought along side Jimbei,
wouldn't he have some knowledge on Haki?
Know about it is different than knowing how to use it. What difference it would make that far back, where noone was supposed to be powerful enough to defeat Arlong in East Blue?
What about Crocodile?
Once again, this is a matter of just Oda don't throwing information before the right time.
And the sword attacks where the force of the attack travels away from the blade seemed like they were thought of later. Why didn't we see any of that when we first saw Mihawk?
Mihawk cutting Krieg's ship, Boggart's cutting Cobby's guns in his coverstory, Mr. 1 cutting things without making contact or even Axehand Morgan cutting a fence far beyond his axe's reach are clear signs that "improbable fencing skills" were destined to appear in One Piece.
@GiantGuy:
I think the intimidating aspect of Haki (Haoshoku) might have been planned from the very beginning. But Busoshoku Haki could easily be an afterthought. Blackbeard is supposed to have been on a ship many years with several capable Haki users. He is supposed to have seen Whitebeard, Marco and the others hurt Logia users. And yet he bragged in almost a chapter on how powerful, evil and unique his Yami Yami no Mi was. A bit curious…
And yeah, I know Oda's the master of Chekhov's Guns, foreshadowings etc., but he is not some perfect god. In any major work, there will be afterthoughts and plotholes.
And yet in Blackbeard's first outing in Jaya, long before we knew about his Fruit, he mentioned Haki. He clearlyknew about it all along.