When he was killed, his body was used to construct the cosmos. With this backstory, it is possible that Ymir may be related to the origins of the Titans or at least how they began to spread across the world, eventually causing the reduction of human population.
Fans and viewers of “Attack on Titan” and readers of its manga source material will recall that Ymir spent about 60 years as a Titan before he regained his human form and the events before his transformation have not yet been explored in the anime series.
In the manga, Ymir has not made an appearance following the “Clash of Titans” story arc and it was presumed by readers that he was already dead.
However, with the recent statement of Isayama, it’s clear that the readers of the manga and viewers of the anime have not seen the last of Ymir.
It is very likely that Ymir will appear again in the future as the origins of the Titans and it shall be continuously explored in the manga and anime series together.
Ymir also did not hold much importance in Season 1 but his being kept in the dark may be on purpose and is meant to allow his character to get the spotlight he deserves when Season 2 possibly launches later this year or early next year.