Actually an upcoming Black Widow prequel film might be the reason they didn't show a funeral scene for her, now I think about it.
I'm surprised they are doing an origin story film for her, because Solo was a Star Wars prequel film for a character that had already been killed off, and that film flopped.
I still think there is a chance that there will be a post-credits scene in the Black Widow film where a portal opens up and Nick Fury or someone comes out and says, 'come with me, we need your help'.