Lelouch has been confirmed to be returning in "Code Geass" Season 3. Although details on how he will return and how he survived his death in "Code Geass: Lelouch of the Rebellion R2" remains untold, speculations of his immortality because of the Code proliferates online.
The release of "Code Geass" Season 3 has been announced by Sunrise during the 10th Anniversary event of the anime. The new anime title of the follow-up is called "Code Geass: Fukkatsu no Lelouch" or "Code Geass: Lelouch of the Resurrection" for its English translation, thus, emphasizing and hinting the resurrection of the main protagonist.
Although there are still no specific details on how Lelouch's return will commence, anime director Gorō Taniguchi commented that the upcoming project would present an alive Lelouch and not as an alternate setting. According to AnimNewsNetwork, Lelouch will return not through alternate dimension version or through the process of cloning.
In retrospect, prior to the announcement of "Code Geass" Season 3, anime fans already speculated that there still may be a follow-up sequel of "Code Geass: Lelouch of the Rebellion R2." According to social media post and video uploads, Lelouch is still alive and his death is but a part of a larger plan.
In view of Lelouch's return next season as his original self, a YouTube channel NextLevelNoobs examines the possibility of Lelouch's immortality. And this somehow aligns with the undying speculations that Lelouch already obtained the Code, thus explaining his immortality. Apart from that, the recently decrypted code, "Rebirth of the Demon," coming from invitations of the 10th Anniversary could also play a significant part with the return of Lelouch next season.
Be it as it may, there is still no certainty on how Lelouch will emerge the next season and how he survived the stabbing from the previous season. There could be a great magical power involved in "Code Geass" Season 3 as what Comic Book pointed out, but guess we would just have to wait and see how things will turn out the coming season.