This isn't a rehash of the coupon conspiracy although it is inspired by the discussions. But the gist is to get people to discuss good ways that Gazillion could get people to spend for the first time.(incentives to bring back players or get players that haven't spent in a long time to spend is a different beast).
NOTE: These suggestions require tech improvements to the store.
1) Non giftable welcome pack(one time purchase and there could be many to choose from)- My opinion is that the best way to ensure that the benefits go to first time spenders (and not dummy alt accounts) is to ensure anything they buy in the incentive package isn't giftable. Otherwise the system would be gamed. The welcome pack should offer a great deal on a few heroes bundled with their stash and general stash.
2) Cash shop that provides credit for things already owned - I'd suggest that the devs in charge of the store look closely at the Heroes of the Storm shop. They reduce the price of packs based on each thing in the pack that you already own. It's slightly more complicated in MH as a hero may represent the hero itself, their default costume and stash. but that's fairly straight forward for them to assign values to.
3) BOGO should be for buy one get one of equal value of your choice - I know some are against this as they want to buy a cheap thing and hope the free one is better. But BOGO should never leave a sour taste in a customers mouth, especially if you want to keep them as customers.