I guess you can say that the only limitations of a tuning is really the player, but for me there specific things I think makes a tuning most useable and versatile. I have to have certain intervals: min3rd, maj3rd, 4ths, b5 or #4, maj2nd. These intervals give me the most of what I need.
Please forgive me for sounding like a broken record, but there are always new players who are looking for a tuning to get started with. C6 is often a turn-off for some players because it is a little confusing at first for guitar players, and some other perceive it as being for Hawaiian only (Brad, you should remove that little quote "everything sounds Hawaiian" from your description on the Page of Steel
When I started out, I was trying to invent my own tunings, if only because the only tunings I knew of were G and D. I wanted something that sounded a little more nebulous (the major 3rd really bugged me because i couldn't avoid it) and which could be more versatile. But the more I did that, the further away I got from learning. At some point I realized I needed to put the time in to get my right hand a bit tighter so I could be in control more.
There are a lot of other tunings that I really like, personally: E, C#min7, E9, E13, A6, B11--all of the proven classic tunings. If you listen, there are guys playing the crap out of all of them. A6 is one that is excellent, but personally I have one issue with it: I prefer to have a tritone interval in my tuning, such as in C6/A7 where the lower C gets tuned up to C#. If you do this in A6 (tune the low A up to A#), I feel like the note is too low and less useful. A6 can be very muddy on the bottom.
E9 (E B G# F# D B G# E) is a great tuning, too, because you can play a lot of riffs on the bass strings, which makes it excellent for Rock and Roll, Rockabilly, etc. Also, strings 2-4 are the same structure as strings 2-4 of C6, just 1/2 step lower.
E13 is also excellent for chordal work and single note playing (E C# B G# F# D B G#--there are several variations on the bottom strings). You can get a great variety of chords for jazz from these two tunings, including quartal chords (built in 4ths), 13ths, #9s, min7b5, etc.
I miss being able to get quartal chords in C6/A7, but with 2 necks (C6/A7 and E9 or 13), I have the bases covered.
Like I said, the possibilities are seemingly endless. It really comes down to you and what your preference is: not everyone sees things the same way. But my one piece of advice is to give it time. You will not get it overnight. The process can be very frustrating, especially if you already know how to play another instrument, but it takes a good solid year or more of dedicated practice.
I haven't been doing much recording lately, but here are a few examples of what you can get from C6/A7. These are Steely Dan tunes that were played exclusively on steel in that tuning, both rhythm and melody:
Josie
Deacon Blues
I don't want to come off like I'm tooting my own horn--I'm not. It's just that most will agree that Steely Dan chord changes can be pretty sophisticated and this proves that it is possible in C6/A7.