Best way to extend Marketpress

For small shops (less than 100 products) MarketPress is great, but there are some options that I see for heavier platforms like Magento that would be nice to have in MarketPress, or be able to develop independently.

Specifically

– Gift Certificates

Not the same as coupons, but something where users could log into an account to check balances and the code or number could be unique to a specific person

– Subscription/recurring products

For things that don’t rise to the level of membership, or physical recurring items that you might want to sell alongside standard products

I know these might not be useful for everyone, and might not make it into a core update, but what would be the best way to develop them without altering the MarketPress core?