There's nothing like a completely open device if you really want to tweak things and flash all sorts of ill-advised software. HTC isn't going to make it easy, though. Getting s-off is necessary to do all that fun stuff, and now the recently released Firewater tool has been updated with support for the M8.
All the necessary files and instructions can be obtained on the Firewater site. It's a fairly straightforward process, but you'll probably need to make sure the device is temp-rooted with jcase's WeakSauce M8 tool first. Once the exploit completes, you should have an unlocked bootloader and s-off. That means permanent root and ROMs are in your future.
Keep in mind that you could very well break things mucking around with the software in unapproved ways, and people will probably laugh when you ask for help fixing it.