This is what I think could be the best possible way Rayman could be revealed as DLC:
It's E3 2015, and Nintendo has announced a bunch of new games. Chief among these was a partnership with Ubisoft to develop three exclusive games for the Wii U: The unreleased Wii U game Ubisoft had developed, ZombiU2, and a complete HD makeover of Rayman 2 would be coming to the Wii U on September 1st, marking 20 years of Rayman. Rayman Advance would be coming to GBA Virtual Console directly after their presentation as well as Rayman 3, making it the first GCN game on Virtual Console. All of these games, Nintendo says, are a sign that their relationship with Ubisoft is getting stronger.
Later that day, Ubisoft kicked off their conference with a montage of Rayman's appearances while an orchestra played Rayman music(think the Zelda 25th anniversary). At the end of the opening, Yves Guildmot revealed that an orchestra at the Nokia Theatre would be performing Rayman music the next day to celebrate 20 years of Rayman.
The next day, a giant banner was hanging on the front of the Nokia Theatre that looked strikingly similar to this:
Underneath the words "Rayman 20th anniversary" were Rayman's appearances from 1, 2, 3, Origins, and Legends, situated on a purple background.
The symphony kicked off with not an orchestra, but a rock band, performing "Rayman's Theme" from Rayman Arena to great acclaim. After the orchestra performed medlies of Rayman 1 and 2, the band performed "Madder" from Rayman 3. After the concert ended with medlies of Origins and Legends, Michel Ancel came on stage to welcome.....
Masahiro Sakurai.
Sakurai explained how Rayman set high standards for not just 3D platformers, but for 3D games in general. As he leaves the stage, Sakurai says: "What you're about to see is a dream that will finally be realized. I wish Rayman a happy 20th birthday.... on the battlefield!"
The trailer plays and the rest is history.