TLOU is my favourite ever videogame and I am so excited for this sequel!
The following is pure speculation on my part about the sequel, but please don't read it if you've not played the original, or just want to go in to the sequel without any pre-existing ideas of what may have occurred between the 1st and 2nd games
I believe that Joel is already dead, most likely murdered by the fireflies because of his actions at the end of the first game. Ellie is hunting the fireflies down in revenge.