Referral Cap in Marvel War of Heroes
Since there is a cap for referrals many people aren't able to get the rewards anymore especially the referral card. But I'm sure that you'll change the card reward for referrals someday, and because of the trade/gift restriction of the referral cards, people who already reached the referral cap, won't be able to get these cards. That's why I want to suggest to either lift the referral cap or lift the trade/gift restricion. So far as I know there isn't a restriction for the Black Widow and the Jean Grey referral card reward, so why is there one for the Invisible Woman card?
Another thing you can do is to reset the referral cap for each new referral card you introduce. With this people are still able to get the cards and complete their catalog.