This game got way unbalanced... I say you should stick with picking decks. Customizing a deck can make it kind of cheap, and kinda annoying picking the deck out. I rather play Card Shuffle 3.2 then this... cause people normally wont pick cards that are crappy like, light heal, ect.
If your going make it kinda like you buy cards, it should have a way to earn money, and maybe cards get upgraded over time, like level up.. but still I say stick with the other version and add like more cards or decks, maybe bigger map, more terrain, 2 more teams, creeps for a challange, weather effects, ect.