I joined the Apple Developer Program and went through the whole process just so I could make iMessage stickers of my cat, the frequently-posted-in-comments-here Zzyzx the Bengal! [more inside]
MusicMessages! is a collaborative step sequencer for the iMessage App Store (iPhone, iPad). Using the simple and (hopefully) intuitive interface, you can punch in a few chords or a percussion line and send the message off to your friends, who can then make their own changes and send it back. Five instrument layers and over 40 MIDI instruments are available to use. Bonus: if you have an iPhone 7, the note buttons respond to pressure and "pop" like bubble wrap with the help of the Taptic engine!