As of now, there’s nothing we know about how it would affect the overall series, but from a fan’s perspective I can tell you that she’ll always be remembered for what she brought to the anime.
For those of not familiar with Shingeki no Kyojin, it is an award winning anime created by Isayama Hajime. The man first ran in Kondansha’s Bessatsu Shonen Magazine in 2009, as of 2018 the series has over 25 volumes. The story revolvs around Eren Jaeger and Mikasa Ackerman in a world where humans are getting extinct thanks to man eating creatures called Titans.
Why did Eren kill Sasha? Why not someone else?
If you asked me this question at point blank range, then my answer would be along the lines of ‘that’s what Isayama-san wanted’ . But let me collect my thoughts and answer this from a fan’s perspective-