I think the whole thing about Shanks wants to talk to Whitebeard (or vice versa, whatever) about Ace and Blackbeard is because they're related.
I LOVE Ace (luvluvluvluvluv) but I'm not sure where his loyalties lie in relation to his little brother. Still, ignoring the characterization of the anime (cuz yeah, they're SO accurate, bleh), Ace would seem to be a very devoted older brother. He went through a lot of trouble trying to find Luffy (the message in Drum, hanging around in Alabasta until he found him) and then when he DID find him was not only overjoyed to see him but also saved his ass from Smoker, and invited him to join Whitebeard's crew (even though he figured Luffy would say no, he still made the gesture, and that's important). He even gave Luffy the Mysterious Piece of Paper which somehow assures that they'll meet again at the "top" or the "pirates' summit" depending on your translation preference. I think the reason he rushed off was because he had spent so much time looking for Luffy that he was afraid of losing Blackbeard's trail. He didn't really have a choice about cutting the reunion short, especially since Luffy and his crew had to get the heck out of Dodge now that the Navy had discovered they were in the area.
I think it also served the plot, since Oda was about to launch the long-ass Alabasta arc and a mini-arc with Luffy's brother would have detracted from that. It was there to introduce Ace as a player for later, and to also make the readers go WTF we know nothing about Luffy's family at all! Which is an important plot point since those references to Luffy's grandfather and "the blood of D" come pretty often after that.
As far as Ace guiding Buggy to Luffy's whereabouts, I have my own theories about this:
- He's messing with Buggy to throw him off of Luffy's tail.
- He's unaware of Buggy and Luffy's history (unlikely but who knows? Maybe he was sleeping through Buggy's earlier rant and only knows that Buggy knows Whitebeard and is looking for his brother).
- He doesn't think that Buggy could possibly be any threat to Luffy.
Whatever the reason, I don't think Ace's intent is to kill Luffy. If he wanted to, he had ample opportunity in Alabasta to do so. If he could blast five ships into oblivion with one mightily awesome HIKEN! he could have totally taken Luffy out at that point. Or yanno, let Smoker or those five BW ships do it for him.
Despite all that, do I think an Ace vs. Luffy fight is inevitable? Hell yes! Whitebeard and Luffy are on a collision course since they're going for the same goal and we all know they're both strong enough to reach it. Ace fighting anyone but Luffy would be a total letdown. Oda's already laid the groundwork for it in Luffy's "I could beat him now, for sure" comment back in Alabasta.
Plus, it'd be totally sexy. >:3
Not to make this post any more long-winded than it assuredly already is, but has anyone else noticed that meat has the opposite effect on Ace than it does Luffy? Luffy famously uses meat to get all energized when he's feeling weak. But every time Ace eats meat (or eats at all it seems) he drops straight off to sleep like a narcoleptic.