The usual Japanese Designer (Hisashi Yuki) has been tweeting something cryptic again, and this time it is of Gen1 Bruticus with Groove as the arm instead of Blast Off.

So what does it mean?

The most likely thing is that TakaraTomy are continuing with their tradition of making sure their Combiners are Gen1 accurate, and Blast Off will be a space-shuttle instead of a fighter jet.
Or, they are having Groove recoloured into Blast Off...

It might be some time before we find out though, as this is an early 2016 toy for Hasbro, so it isn't likely to be released in Japan until some time after that.