As noted by Autoblog, the tweet is a naturally spartan form of communication, even in Japanese. But our favorite machine translators tell us that it's not so much the character count as it is the official stance that's limiting the content about the next rotary.
The tweet starts with a thank-you to fans of the RX-8 and the rotary, and goes on to say that Mazda is "zealously developing" the next generation of the rotary engine--with the benefit of its Skyactiv fuel/emissions technology, closing with another thanks for the continued support.
What does that mean? As near as we can interpret, that means: 1) don't hold your breath--even if it does come, it's still in raw developmental stages; 2) the rotary is being hamstrung by its inherently poor fuel economy and emissions; and 3) if you're a rotary fan, Mazda thanks you. A lot.