I hope they avoid the thumbprint thing. It's unnecessary and stupid, with potential for things like nefarious collection of such information for no reason. Entering numbers on a keypad isn't that hard (though I don't even use security on my phone because there's nothing valuable on it).
As for what I'd want...nothing. The 1530 doesn't suit me because I don't find it to be portable enough. However, I'll say that I'm not in-favor of a metal body because they conduct heat too well (meaning it can get annoyingly hot and cold to the touch when left in a hot/cold area), and it would likely be similar to the One (M8), where the metal is too slick to use without a case, meaning t he more-expensive (and not-more-durable) metal chassis is hidden from sight. So, I'd prefer sticking to polycarbonate as the main body. I also don't care for that metal band on the 830/930. I know it's supposed to be used for improved reception, and I could live with it, but I don't find it aesthetically pleasing.
On the internals side, I have such low expectations from Microsoft that my wants are few. I'd want a Snapdragon 810 and an improved 20-MP camera. 32 GB of built-in storage, preferably with microSD storage included. I'd want an IPS display at 1080p, as anything higher is an unnecessary drain on the battery, in the name of pushing pixels that are mostly indiscernible. However, Microsoft showed up with the 830 that they are cheap, and we might have to settle for a crappy 805 (compared to the 810), 20 MP camera, and 16 GB of on-board storage.