HTC is currently under huge loss and there are chances of closing the smartphone business anytime soon. Now we have spotted one of the HTC Senior RF profile on LinkedIn who has mentioned the company is working on the successor of SD845 SoC.
So HTC is planning for the successor to HTC U12 Plus which will arrive with the new SD855 SoC. Apart from that, the profile also mentions the device might also come with latest Snapdragon X50 5G Modem which is part of the SD855 SoC but will be advertised separately as the chipset will come with Snapdragon X24 LTE Modem.
Anyway, HTC is now working on the 2019 flagship as we all know HTC financial condition is weak and the company might not launch another device anytime sooner than 2019. So HTC will probably launch the HTC U12 Plus successor not earlier than 2019.