Most important IMHO :
-More cards. Each element should have at least 30-50 different cards.
-Change Password option in Settings.
-Don't allow logging in if someone is already using the account, as that can lead to losing cards in the current system. (most of the time, yourself in another browser windows, but also when a family or friends play on a shared account)
-Card Trading. Rare cards are not only rare, it's completely random which one you get. If you can't even trade for it, unless you are lucky, your chance for obtaining the specific rare card you need is nearly zero.
-Changing the reward cards in top 50. The current system of rewarding the cards in the player's deck gives control of the spread of rare cards to the top 50 players. It would be better if the cards were given out somehow based on the win/loss ratio of the player while he was using THAT deck. Like, WIN/Loss ratio of 40% or more wins against false gods, pvp or top 50 with the deck= chance for upgraded card and rare card, 20% or more, chance for rare card only, 0-20% only common cards.
If the player didn't play at least 5 matches with the deck yet, he won't appear in top 50, and the next player (51th, 52th etc) is chosen. The reward cards are not chosen from the cards in his deck, but from all cards existing in the game altogether.
-Reduced cost on upgraded cards or increased amount of gold gained from duels. If there are more cards, it takes a lot more time farming to get them. This is a card game, grinding shouldn't be THIS hard.
-Have more control over which false god you play. Not necessarily by selecting which one to play directly, but a button when you lose against one for "stay away" perhaps for some additional electrum cost(30-50 electrum at most), which would prevent that god from facing you the next X (around 50-100) times you play against one. (You could have up to Y (3-5) gods selected this way) Most decks have at least some gods where they don't stand a chance. Alternatively, allow sideboarding in false god matches, but I think the stay away button is better (and easier to implement most likely).
-Allow more than 8 cards in hand. It's very very annoying when you have quanta to draw extra cards from your hourglass, are in desperate need of more pillars because you can't play a thing, and you are unable to use your hourglass because your hand is full of cards you can't play yet.
-Ways to get back cards from your graveyard/discard pile..oh wait, there isn't one in the game...make one. Not being able to see which card are already used up by the opponent isn't very good anyway, having a graveyard would fix that.
-More quests in increasing order of difficulty that reward rare cards for every 3-5 quest you do, like
1.Obtain a rare card by trading, 2.Defeat a false god, 3.Win 10 times in a row against a level 3 opponent, 4.Win 10 times in a row against a level 4 opponent, 5.Win 3 times in a row against a false god, 6. Win 5 times in a row at level 4 (Top 50), 7. Win 10 times in a row against a half blood, 8. Win five times in a row against a false god, 9 obtain a score of 100,000, etc...
presumably by conning newbies out of rare cards, or funneling several accounts' cards into one
That's normal in all card games. You must know the value of the cards you are trading, it is your responsibility. As long as the game makes the rare and the upgraded cards properly distinguishable, (for example rares can have "rare" written on the in red letters in the trading window, upgraded ones already have a different border color so those are fine). If the trade doesn't include rare cards on a side (or both sides), and that side's card total buy cost is less than the bazaar sell price of the other side, the trade can be automatically canceled by a message of "Doing that trade in the bazaar is a better deal".
Generally, trading can only be allowed for rares, or where neither side loses out for cards with a well defined bazaar buy/sell cost. So basically all you could do in the "conning" department is getting a better rare, or multiple rares for a worse one, and I see nothing wrong with that. The person trading might need the worse rare that much to actually want to trade that way, making the actual "value" of these cards very subjective.