These are some features I've seen in another game that I think you should implement some variation of into your game.
1. VIP sign in bonus daily. Vip 0 gets 25 rubies everyday they sign in. VIP 1 gets 50 rubies. VIP 2 gets 75 rubies. Vip 3 gets 100 rubies. Vip 4 gets 100 rubies and 1 diamond. VIP 5 gets 100 rubies 2 diamonds. Vip 6 gets 100 rubies and 3 diamonds.
This is huge as it incentives two things. 1. It incentives to buy VIP and that is something that costs money for good VIP. Many new players will quickly buy VIP to get the boosted sign in bonuses so they can progress in the game faster. 2. This creates a desire to login everyday. You should also reward argent card every 30 days or something with VIP 3 or 4 maybe. This could get more people paying into the game. Aside from this a free player should still have some way to get that free legend too such as a "launch" phase. Maybe the first ten days if they login ten days they get one free random legend as a one time deal.
2. Raid mode. So this is something I've seen in ankther game that's a card game but different than this one. Every day say you are allowed to raid another player within say 30 ranks of you on ladder. Each player who plays a ladder match is prompted to setup a raid deck, this is just six cards that they will use if they get raided. Everyday other players can raid other decks. It's sort of like campaign mode in the sense that you will play against six known cards. This gives lower players the chance to beat higher rank players bc they are able to devise the correct strategy to beat them. This will do tremendous as it shows that any deck can be beaten. You can award 200 gold and 20 rubies for completing five raids each day. No player loses anything for being raided. Whats nice is that the player being raided does not have to be online at the time bc they have a raid deck set up as their defense deck against other players raiding them. You could also play out raid mode like a pro mode to make it more difficult. Such as you only get to see two cards per round.
Anyway this is just another feature of the game and could be incorporated into guild mode. It also gives players another way to play the game and the chance to gain confidence by beating better players. It will teach them to understand the game better as no they have an unlimited number of "campaigns" to play against and they can study the deck and lick the right combinations to beat them. This would be a fantastic way to show new players that they can beat decks with epics by outsmarting them and help them play REAL decks that are actually trending in the current adder rather than the fake decks you put in campaign mode that are unlike anything you will ever see in a ladder match.
I hope you consider these suggestions. Both are designed to get and keep new players as well as incentivize them to make small purchases. I hope to help grow the game.