Here's what we know so far:
Snapdragon 805 processor @2.5 GHz
5.5" QHD Screen (Super LCD3 capacitive touchscreen with 16M colors)
I've also read 5.0" screen (unlikely)
Internal Storage: 16 GB
WATERPROOF
High speed Cat. 6 LTE radio
Up to 128 GB SD card storage
16 MP UltraPixel rear camera with depth sensor (updated by EarlyMon, TY) Link to Early's Post
Composite of aluminium and liquid silicone!
"One of the probable selling points of HTC M8 Prime is the metal casing of the handset. The new handset will not pack an aluminum casing but a composite of the metal with liquid silicone which offers more benefits than aluminum such as better durability, higher thermal conductivity and cost-effective production."
Reports claimed that HTC M8 Prime will pack the next generation of Android OS dubbed as 4.5 Lollipop. What is Lollipop?
(Release date 3rd Quarter (Anytime June through October?))
Rumors indicate 3rd quarter if it is a One Max update, but as soon as June (unlikely) if it is an LG G3 competitor.
From @evleaks:
Phandroid Article
Message To Potential Insiders: Please feel free to PM me with any leaks you may have. I assure you your identity will be closely guarded. (I'm quite experienced with handling leaks and secret identities, even when tortured! )