I think a more durable black coating on the clips sounds like a good idea. I like black clips.
Making a clip less prone to snagging on things like seatbelts would require closing the gap at the end of the clip. This would then make repocketing the knives more difficult. I have had a Spydie with a small gap at the end and I took it off and bent it a little. If you are having issues with it snagging just tweak the end a little. This is something the knife owner can easily adjust themselves.
If it is more retention that is desired then there is always the option of bending the clip to make it tighter. I have tweaked a few clips this way to make them either a little looser or a little tighter. I find that the scale material makes a difference too. Peel ply G10 can give great retention while smooth metals give less retention. There is also always the option of removing the clip and scuffing up the contact surface on the back of the clip but that can increase the wear on your pants. Tension and traction both seem adjustable.
Deep carry is nice for the aesthetic reasons. In dress clothes it makes sense for a gents folder to be discrete and classy looking but for EDC I just do not see the need. Deep carry, for me, makes extracting the knife form my pocket a little more difficult. I enjoy having a little bit of knife sticking out to ease extraction. Also, I do not feel as though I need to hide my knife from anybody. I do not care what others think of me carrying a knife and I do not want to live in a world where we feel the need to hide a basic tool.
Milled clips seem to be gaining popularity and while they do look nice and I am sure they are sturdy they are not without their downsides. They cost more to make and that adds to the final cost of the knife. They are also more difficult to adjust like I mentioned above. For this reason I prefer a stamped clip.
There is another trend I am seeing lately and that is placing a single ball bearing into a clip at the contact point. I assume this is to ease extraction and to prevent wear on pants. To me this is a little gimmicky and is totally unnecessary. It probably reduces retention and seems like a solution to a problem that doesn't exist. It is a cool idea but is basically just there to be cool. I feel as though this is best reserved for pocket jewelry.
I am not against continuing to improve and always looking for better ways to do something but at this point in time I have not seen much that is an improvement on the standard hourglass clip in my eyes. Sorry for being so long winded and this is obviously just one man's opinion.