@Fire Fist:
Maybe a little grasping at straws here, but I was re-reading some chapters and thought of something: What if Ace was hired by Moriah to retrieve Shusui and Ryuuma's Corpse from Wano country? This would explain a couple major plot points:
1. What Ace was doing in Wano country. His goal was making Whitebeard Pirate King. Going to Wano wouldn't accomplish that goal.
Neither would reuniting with Luffy or hunting Teach.
It's not like every second of Ace's time was singularly devoted to making Whitebeard the Pirate King.
@Fire Fist:
2. Why he made the hat for Little Oars Jr. He could have seen LOJ's ancestor's corpse (and thus learned he died to exposure) while talking to Moriah, and most likely would have felt bad that the original Oars was being used as a massive meat puppet. So, to keep LOJ from repeating the same fate, he made the hat so he wouldn't be exposed to the sun.
It just seems like Little Oars Jnr liked sitting out in the open.
There doesn't seem to be any indication that the line of Oars was anymore susceptible to dying of exposure than anybody else.
@Fire Fist:
3. His behavior during the war. Specific things: Him being the only one shocked that Luffy is Dragon's son, as if he had worked with Ace before, it would be likely that he heard Ace refer to Luffy as his "brother",
Quite a few people were visibly shocked by that revelation, not just Moriah.
@Fire Fist:
His delight in seeing Whitebeard dead (if he owed money or a debt to him for retrieving something like a corpse, his death would eliminate that debt).
Or he's just being sadistic as usual.
@Fire Fist:
4. Kinemon accusing Zoro of being the one who stole Shusui (And Ryuuma's Corpse). Besides the obvious "Possession is 9/10 of the law", Zoro and Ace are roughly the same age. If they had reported a young man of about 20 or so had taken the corpse of their national hero (and thus, his sword), it would be easy to be mistaken identity, especially if Ace wasn't seen doing it and they had no visual reference to go by.
How could they report about a young man of 20 if Ace wasn't seen doing it?
Also Kin'emon seems to have been set off by the sight of Shusui alone and made the connection, if Zoro also fit a description of the possible thieft, wouldnt Kin have mentioned it?
@Fire Fist:
Again, I'm probably just thinking way too far out of the box like I always do, but I'd like to think that Oda had some kind of subtle hint with Ace's random friendship with a massive giant who coincidentally is the ancestor of the one his brother Luffy beat a few months before hand.
I think you are indeed thinking way too far out of the box buddy.
Oars Jnr. was basically brought in for the dual function of 1) Showing the power and variety of WB's allies and 2) Showing that the line of Oars didn't die out.
Adding in a friendship between Ace and this humongous battering ram that was always going to get brought down during the war just added to the emotion of the entire conflict.