There are tons of payment gateways out there. Is it not possible to develop them once and then add them to every single plugin where a payment is needed. Maybe you can put them in a payment plugin, this would save a lot of extra work and would make all the payment gateways instantly available for every new plugin.
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http://wpmudev.uservoice.com/forums/148158-wpmu-dev-new-development-requests/suggestions/2721193-isn-t-it-possible-to-add-all-the-supported-payment?tracking_code=3f7f29eaa07199fb4c65eb301ad36f8f
Suggestions are always welcome!