According to The Verge, OnePlus will be announcing its next phone at some point in the summer and the successor to the OnePlus 3 / 3T will not be called OnePlus 4 because that number is considered unlucky.
Going with the earlier rumors, OnePlus 5 is expected to a 5.5-inch display and will be powered by the Qualcomm Snapdragon 835 SoC. It will have 6GB of RAM and 64GB/128 storage.
The OnePlus 5 will sport a dual camera at the back, and one sensor will capture monochrome shots, while the other one captures standard shots. The megapixel count is not unknown yet.
The phone, as expected, will come laden by Android 7.0 Nougat based Oxygen OS. The phone will have a 3600mAh battery and Dash Charge 2.0 will charge the device faster than the OnePlus 3T.
There might be a OnePlus 5T launched as well with 8GB of RAM and 256GB of internal storage and bigger battery, but these are just speculations, so take them with a grain of salt.