OBJECTION!!!
https://www.youtube.com/watch?v=5hx8WTHcie0
If it was trully a Monster Hunter reference, why Link, Marth and Shulk?
http://ca.ign.com/wikis/monster-hunter-world/Weapons
As you can see, Link can fill the Sword and Shield spot, but why Marth and Shulk? Why not a character who as an Hammer like Dedede, a bow like Pit, a GreatSword like Cloud or a BowGun (Samus)?
I'l tell you why, this is no Monster Hunter reference.
Beside in Monster Hunter you can play up to four player, why not add a fourth one?
I heard Sakurai doesn't talk a lot, I feel like every single of is tweet must have a meaning behind it, promote the game or tease.
While it's not definitive proof, it add to a lot.
Sakurai is 100% teasing Isaac for those reason.
First we have 3 characters in an arena, in GS, if you play against a friend, you can only use 3 characters instead of 4 since the GBA can't handle 8 adepts (apparently).
Second, why Link, Marth and Shulk in this order?
They are shown from oldest to newest, and we can replace them by your first 3 party members in Golden Sun.
Link = Isaac, both are silent protagonist.
Marth = Garet, Marth come from FIRE Emblem and Garet controls fire.
Shulk = Yvan, both can see into the FUTURE.
+Someone pointed out that out of these 3 characters, there is no Brawl rep, just like in the assist trophy list, Isaac Trophy is missing, Isaac could be that Brawl character.
Beside, this angle is rarelly used in RPG, Camelot is one of the only studio using it.
What a coincidence...
Also, I believe that Rathalos may be a reference to the Fusion Dragon, the final boss in Golden Sun, why?
Both the Fusion Dragon and the Rathalos use fire attack and both can poison.
Beside the fusion dragon comes from the fusion of two Mars adepts (mars adepts controls fire like Garet)
And in Monster Hunter, the Rathalos drops can be use to make Fire Weapon, while the Rathian (the female) is use to make poison weapons.
So Rathalos is more link to fire then poison just like the Fusion Dragon.
Beside it's not the first time that Sakurai has use Twitter to tease a character, he did it whit Palutena, Shulk, Bayonneta, and Simon/King K Roll (red eyes)
He even put a freaking red and yellow barell during E3 were he was getting record to tease King K Roll early, so it would not be the first time.
Finally Golden Sun Dark Dawn page was update twice in the past weeks, it was the first time in years. (It may very well be nothing thought).
It's a little too much in my opinion to be call farfetched.
So do not lose hope! The sun always rise in the sky!