OnePlus already started designing a OnePlus 2 successor, but its next phone won't be called OnePlus 3, nor OnePlus 2S9
During a Reddit AMA (ask me anything) session, OnePlus told fans that its next smartphone will not be a "OnePlus 3", and neither a "OnePlus 2S". In other words, this is not a OnePlus 2 successor. It might be the rumored OnePlus Mini, since the company also mentioned that it's going to "broaden" its range of handsets this year. However, for now we can't be sure of this.
So the new smartphone that's bound to be launched later this year isn't a OnePlus 2 successor. But this doesn't mean that OnePlus isn't already thinking about a successor. In fact, the company says that the design process for its third-generation flagship has already begun (and that's all it can say about this for now).
OnePlus also stated that it's planning to continue selling the OnePlus One (which is now over 1 year old) throughout 2015. After that, though, it looks like the handset will no longer be available to buy - which makes sense, since a brand new device should soon join the OnePlus 2.