It seems that the E9 will be HTC's first smartphone to sport a Quad HD (1440 x 2560 pixels) display - one that measures 5.5 inches. However, unlike the M9, the One E9 won't use a Snapdragon processor, but a MediaTek one: a 64-bit, octa-core MT6795 processor clocked at 2.0 GHz. The One E9 should run Android 5.0 Lollipop with Sense 7.0 UI, while also featuring: BoomSound speakers, a 20 MP rear camera, 4 MP UltraPixel front-facing camera (or 13 MP, on some variants), 3 GB of RAM, and 32 GB of internal memory.
It's said that the One E9 will be available in three color versions: white rose gold, metallic gray, and brown gold. All in all, this could be that One M9 Plus that you've been expecting, though it's not clear if HTC is planning to launch it around the world. We'll let you know when we find out more.
P.S.: What you're seeing in the image above is just a concept design, not a real device.
source: @upleaks (translated)