Super Smash Bros. for Wii U is a Nintendo fighting game released on November 21st, 2014 for the Wii U console, and serves as the fourth installment in the Super Smash Bros. franchise. It features over 50 Nintendo characters from various Nintendo-owned properties, as well as several third-party characters.
There is a much smaller amount of Golden Sun content in Super Smash Bros. for Wii U when compared to Super Smash Bros. Brawl. The assist trophy for Isaac that was present in Brawl has been removed, as has its companion trophy. The reason for the removal of the assist trophy and its companion trophy are unknown and have not been officially commented on by any member of the Golden Sun or Super Smash Bros. development teams. There is no evidence from data mining that these pieces of content were ever intended to be present in the game.
There is the potential for future content from Golden Sun to appear in Super Smash Bros. for Wii U, as well as Super Smash Bros. for 3DS. On April 1st, 2015, a Fighter Ballot for DLC fighters in both games was announced via a Nintendo Direct, for which the voting period concluded on October 3rd, 2015. Isaac is believed to be in very good standing within this ballot, with many fan polls having him in the top five characters requested overall. However, any official information regarding future Golden Sun content in either Super Smash Bros. for Wii U or 3DS has not been confirmed by Nintendo at this time.