I'd take a Wii U Golden Sun, mostly because I could play on the Game Pad. I'm really just a much bigger fan of handhelds these days.
I think our biggest hope for Isaac is the Assist Trophy. I mean, we know that Sakurai didn't pick them willy-nilly. We know he thought about the characters hard. When going back to make a new Smash Bros. game, he has to look at them all again, right? I mean, he pulled one for promotion, surely he'd at least look at the others.
Of all the Assist Trophies in Brawl, what is unique about Isaac? He has a defined play style. So many of the Assist Trophies can't really do anything else. I mean, how would Excitebike work as a fighter? What would Andross do? Or Hammer Brothers? But Isaac has moveset potential. It's frelling obvious. Far more so than any other Assist Trophy. Definitely more so than Little Mac (though he does admittedly have less demand). If Sakurai did look back on the Assist Trophies, Isaac's moveset potential must simply be screaming for a full character.
Right?
Yes, plus Isaac's promotion to a playable character is the only actual promotion he could get. Other characters in high demand like King K. Rool, Bowser Jr, Chibi-Robo or Shulk would maybe become Assist Trophy summons if Sakurai ended up to not include them in the playable roster for one reason or another; obviously their supporters would be upset to not being able to play as them, but it's still a sort of promotion compared to Brawl when they don't appear in battle at all, we already had this situation with Takamaru. But if Isaac is an Assist Trophy again and we don't have another Golden Sun playable rep to compensate that would mean that Masahiro Sakurai feels that Golden Sun deserves no promotion, and that would be really sad (and lol I don't consider an hypothetical transition from unlockable to starter Assist Trophy as a promotion).
In my opinion the only real obstacle Isaac is facing is the lack of Golden Sun content SSB4. I mean, every single newcomer (with the obvious exception of the first three announced) had content from their game teased to be featured in Smash 4: Rosalina had the Mario Galaxy stage, Little Mac had the boxig ring revealed in the very first trailer, Greninja had the Xerneas pic revealed a few day after Pokémon X and Y release, Palutena had A LOT of KI:U content, Mii Fighters had the Find Mii stage, Robin and Lucina had the Arena Ferox stage that was revealed in the very first trailer like the Boxing Ring, plus the Id (Purpose) music revealed in the demo that directly hinted at Robin. The only exception is Pac-Man, but even then we had the S-Flag and Boss Galaga that come from Namco games, so Pac-Man was still hinted at before his reveal, just more indirectly. But at the same time Shulk is one of the most expected newcomers due to a lot of teasing on Twitter and the fact that Shulk's voice actor will probably voice him in Smash 4, but still we don't have any Xenoblade content revealed for Smash 4. Maybe both Isaac and Shulk are going to be secret characters with no pre-launch trailer.
Isaac's situation is so similar to Little Mac's that's not even funny. Both were popular Assist Trophies from Brawl, both represent a specific part of Nintendo's story in their origins (the arcade games for Little Mac, and the Game Boy Advance for Isaac) probably better than anyone else, both got a game a few year after Brawl, and both get high demand but mostly outside of Japan. The only difference is that Little Mac was hinted at very early with the boxing ring to be revealed not very late, being only the fifth officially revealed newcomer when this game will have roughly 15 newcomers. While we have no Golden Sun content teased for SSB4 as of now the release of BOTH Golden Sun games from GBA for the Wii U virtual console definitely helps Isaac's cause and is a strong counterargument against "Golden Sun isn't relevant anymore", because if you think that a series isn't relevant you don't re-release all of its game from its original system for the Virtual Console, especially during the pre-SSB4 period
though Golden Sun 2 needs to be released for the Virtual Console outside of Japan to make this argument really strong.