In addition to Casey coming back, it’s also very likely that Stella Kidd and Kelly Severide’s wedding happens in the finale. Fans are clamoring for a big Stellaride moment, since her absence for a lot of Season 10 kept the couple apart. A wedding would be the perfect fan service.
Casey and Severide are longtime best friends, so from a story perspective it would make no sense if Casey didn’t show up to the wedding, especially since his girlfriend Sylvie Brett would be involved. Fans know that, and Spencer probably knows it too, so we can expect that Casey would turn up, even if it’s just a quick cameo. Everyone involved seems to want it, and it would be the perfect way to end the season.
Source: https://www.distractify.com/p/chicago-fire-season-10-finale-predictions
