Updated after work by @therewasaguy:

XML is the one thing that could really benefit from some more work. Currently, we can load it, but it's hard to do anything with it because there is not yet an XML object like there is in Processing. An XML object could be really useful in tandem with p5.dom. An XML to JSON converter could also be useful even though that wouldn't always translate. I'd like to work on XML in theory, but in reality I'm not sure when I will get a chance, so if anybody is inspired they should go for it (and maybe open up a new ticket?).

More stuff that isn't supported and might be worth adding:

• loadBytes() / saveBytes() --> actually spent a decent amount of time trying to figure this one out. Maybe this will help.
• beginRecord() / endRecord() --> save a PDF of the sketch
• beginRaw() / endRaw() --> could create a vector?