Sony created a design language for their flagship mobile devices called OmniBalance. It is based on symmetry from every direction and it has evolved throughout the years in a subtle way. The Xperia Z5 lineup was announced recently, and it includes 3 smartphones, the Xperia Z5, a smaller one called Xperia Z5 Compact and a larger one called Xperia Z5 Premium, which actually sports a display with 4K resolution. The company was one of the first ones to release two flagship devices every year based on how fast technology is evolving, which turned out to be true, but also, there's not much time to innovate in that timeframe, so users don't really see the appeal of upgrading to the newest generation.
A video from an interview with the Art Director of Sony Mobile, Rikke Gertsen Constein, has been spotted where she explains more details from the design language that has been used lately, plus some details about the future. First of all, it seems like the OmniBalance design is here to stay, for better or for worse. She explains that the company was very pleased with the design of the original Xperia Z and she thinks that it is quite an iconic one, so she doesn't see the necessity of changing too many aspects. Some phones would be distinguished by the materials used on their construction, for example, the Xperia Z5 uses a frosted glass back while the Premium option uses an extremely reflective surface on the back, but the overall design of the two handsets is still very similar.
When asked about the strategy for future devices (around 3:41), she replies that the company is always planning ahead their future designs for the years to come. Particularly, she speaks of phones that will be released on the second half of 2016. This could mean many things, one theory is that Sony will change its current strategy and not release another flagship until a year from now and release one flagship every year in the future. But it could also mean that they would make some improvements to the current Xperia Z5 using an almost identical design and release a Xperia Z5+ on the first half of the year just like they did this year. Other companies have used similar strategies, so Sony could have been right from the start.