I don't know exactly what you mean with a new type of coin? Would this for instance be a coin worth 20? Or does it has a new special feature? There are already 3 types of coins, so why do you want a fourth?
Enchanting armor... Meh, I have the feeling people with diamond gear are already unbeatable, and enchanted armor would only make the ones with the coins more powerfull. In my opinion there is no need for that.
About the armor trading into coins: I wouldn't like to see this in Assassination. When I play Assassination, I have a very hard time getting good armor and stuff (because I'm not that good at PvP). I like it when you are walking in the arena and suddently there is some iron/chain armor on the ground (from diamond-armored-dudes that don't need that armor). It makes it easier for me to fight against other people with better armor. If this would be implemented, all armor would be sold and it would be very hard to compete against these high armor PvP'ers.
Conclusion: I actually just prefer how it is now. Your suggestions (the last 2 ones) are in favor for the ones who have all the gear and coins. I don't like that.
(Sorry if I'm being rude)