A few things to consider:
First, earlier this year, Take-Two Interactive boss Strauss Zelnick said Red Dead was one of their "permanent" franchises.
Second, Grand Theft Auto V is now pretty much complete; Rockstar said they wouldn't be starting on anything new until GTAV for PC was out the door, and that's long since done.
Third, it was just announced via shareholder outreach document (as cited by GameSpot ) that RDR had now shipped over 14 million copies worldwide. Part of the document talks about how the game "rejuvenated the western entertainment genre," and adds that the game's downloadable content was "commercially successful."
Fourth and finally, we've gone an inordinately long time between new Take-Two game announcements. GTAV took longer than usual (when compared to previous installments in the series), and Rockstar hasn't had a huge presence at either of the past few E3s. They're also hiring like crazy , which is a sign of development that is either ramping up now, or will be ramping up in early 2016.
The bottom line is that I fully expect to hear about a brand new Red Dead installment before the first quarter of 2016 is over. Mark my words.