Two months ago I was pretty sure (or hoping) Morgan would be the one, but lately I have been having the same thought. People are getting used to seeing Glenn find some way out of impossible situations. This might lead to many people thinking Glenn will not be the one Negan kills, so the shock value still could be there.
I'm also not so sure that Daryl will be spared either. If the writers want to shock the most people then killing the most popular character would certainly do the trick. Just think how much hatred the audience would garner for Negan for killing their favorite character. This also helps Dwight to evolve into the character he is in the book.
Lastly, my guess is that the final scene for this season will be Negan swinging his bat but not showing who he decided to hit so we won't really know who dies until the start of next season. I hope I'm wrong but I know how shows like to end the season on a cliffhanger.