Retronyms has release Stryke, a strummed instrument app. Tabletop has been updated with a built in version of the app.
Stryke iTunes Description:
Unleash your spirit animal — fame awaits!
Stryke turns your iPad into a real musical instrument. Pluck, strum, shred with expertly crafted tones. Grab the app and a camera and join the community of #StrykeShred users. Cover the classics or compose your own masterpiece!
Stryke is Tabletop-Ready!
Tabletop is a free musical environment for iPad where you can mix and match different instruments and effects. Think of it like the rest of the band.
Select your spirit animal and let the magic unfold. Your ultimate legacy begins today.
That description doesn't do much to describe anything, but this retro video gives a good sense of the vibe!