Definitely the google APIs.
I'm looking into creating plugins relating to managing files or photos with those, and their login system.
Also I might be interested in overall commissioning some plugins and revising them to a really professional level for sales in bundles.
But this is a much more long term goal, as i'm working on various assets that would go together well with plugins, etc. I'm very interested in business marketing and branding. It's not just my hobby to be working on these plugins.