I'm sure many of us with a POD HD500 have wanted to use it via the four cable method or even solely in an amp effects loop to provide FX only. The many threads suggestion how to do this show two things (a) It's a popular way of using the product (b) Line 6 really didn't do its homework.
Getting input/output levels to use either of these FX methods is time consuming and fiddly, when it really shouldn't be. So my suggestion is that in a future firmware update Line 6 adds two modes selectable in the settings menu. One sets up the POD HD500 optimised for the four cable method, the other for use with the whole device within an effects loop. Line 6 has the expertese to make sure all the levels are correct, rather than the trial and error nonsense we all go through.
I've got a good 4CM setup now on my HD500. But it would be so much easier if it was just possible to switch to different modes on the device that did it all - such as removing amp blocks - perhaps giving us more FX slots, or simply just making it easier for everyone. I can't imagine that this would involve much work.