Copy Pasta time...
https://twitter.com/sora_sakurai
I believe that Isaac is in this time, this is just too much...
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.
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 is a Mars adept, so he controls fire.
IMPORTANT: Marth name can be translated to Mars.
Exemple FF7: Aerith/Aeris
Marth/Mars
In the fire Emblem OVA Marth name is Mars.
Shulk = Yvan, both can see into the FUTURE and have an item that the evil guys wants (Shaman Rods/Monado)
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 (Edit: Months id say now), it was the first time in years. (It may very well be nothing thought).
So yeah, I won't believe that Isaac is not in the game until I have a pop up on my Switch telling me: Congratz Everyone is Here... except that blond anime Swordman.
Edit: I wanted to believe that Isaac would be shown today, but this is a nintendo Direct, focusing on a more casuals audience, now Isabelle who was gonna happen anyway is in.
Edit Again: The trademark was renewed.