Very nice.
Its great to see someone who was moved by Cosmo's sacrifice. Theres only one problem with your storyline though, It wasnt really Eggman or Shadow's fault she died. The Metarex wouldve got as far as they did with or without any help. As we all seen, not even Super Sonic or Super Shadow combined wasnt enough to defeat them, it was Cosmo's DESTINITY to sacrifice herself for them to defeat Dark Oak. In the end, it was a very dramatic goodbye, it would even be more dramatic if 4Kids TV didnt butcher Cosmo's death becuase in the original japanese episode of 77 of Sonic X, there was much more drama in her last moments. And they also altered the fact of she would "Actually die" and downsized it into "Spreading her seeds across the galaxy."
Overall, your flash is very interesting and i give it a solid 5/5, keep up the good work =)