Take this with a grain of salt, but famous leaker FunMW2, posted awhile ago on gtaforums in a spoiler, "Next game is a new Red Dead. Two new games scheduled for 2017."
Judging from his past leaks, most of them came to be true. I will find that post and source it. I assume he has sources from R* NYC with one of them being added on Skype.
Aside from this, I'm sure their next game is a new Red Dead. An announcement before E3, then perhaps a trailer at MS/Sony's press conference.
Found the post.