I think you have a great idea! There should be some form of Credit Card purchase of Steem option.
Here's the thing: I live outside the USA and cryptocurrency exchanges are not always willing to work with 3rd world countries and their inefficient banking systems. In fact, I know MANY people that are trying to get in to Crypto trading, but the barriers of entry are ridiculous.

So YES! Please add credit cards. If not through Steemit, then through some exchange.