So just a recap on the Rathalos pic, all the evidence for Isaac we have is as follows:
It directly mirrors the Golden Sun Battle layout.
If it were a reference to Monster Hunter, it'd be more dynamic.
The coliseum in the background is reminiscent of Colloso.
The 3 characters, Link, Marth, and Shulk are organized in order in which they appeared in the series, blocking out a potential 4th "party" member. (newcomer)
Link could represent Isaac as the blonde silent protagonist.
Marth being Mars in japanese, and being from Fire Emblem makes an uncanny reference to Garet.
Shulk is primarily associated with Ivan from reading minds / seeing the future.
Fighting dragons isn't anything new to Isaac.
That kind of pose is rather difficult to replicate without editting.
Sakurai would've likely picked up on the likeness, and not have posted it if it wasn't a tease to Isaac.
Assist Trophies are lined across the right hand side, hinting further about a newcomer that used to be an Assist.
All 3 characters are from different types of jRPGs, with the main type of jRPG missing is a turn-based jRPG.
Other than that i think it's just we haven't seen an Isaac AT, and he's been highly requested for several years. Did I miss anything?