I think a great idea to fill the Reward-o-Tron would be with the system they had I GW2. Although most people did not enjoy the customization recolors of silver, gold, etc. it would give some purpose to coins. Additionally, I think it would be cool if after ever two new characters, they drop a huge content update, not only with the new character, but new customizations for ever character and the recent ones. It would be a breath of fresh air.
Lastly, I think they should increase the prices for new characters to 1 million coins for each one, and have the legendary upgrades also available for 500,000 coins (Or 750,000 coins). I think it is too easy for many people to get the new characters if they don't increase. Most of my friends waited for Wizard to be purchasable because he was so cheap, making the previous prize map somewhat weak (disappointing) in my opinion. This aspect makes the prize map less rewarding in my opinion, although I love how it is shaped and how you get the rewards.