My Opinion
-The ticks on the check boxes are almost imvisible, changing the tick colour would be nice
-Balance may also be an issue, as in to allow some other elements to have access to other abilities, maybe something like a colour wheel...not sure how to explain this, but eg. make another element or 2 able to make invincibility, but more expensive, since aether rules this field they have it the cheapest etc. (I am just basing my opinion off magic:The Gathering), other cards which allow you to draw cards (time rules this atm. so others would be more costy obviously)
-Maybe a better chat system in pvp, messaging is annoying...
- I am not sure about this one but maybe there could be a way to "buy cards" from the bazaar, with real money...or something to give credits which can be used to do so (not to pay to get more gold coins as that would be a little unfair for upgrading purposes) this may also help zanz get more out of his work, however if the donate generates enough for him then nevermind...
these are all i have at the moment
Hope it helps