I’ve been wanting to revisit Rock Band or Guitar Hero these past few years, but the price (even on the secondhand market) and large size of the guitar peripherals have kept me from taking the plunge. Now if they made a compact and inexpensive guitar controller for these games, it would be a different story.
I propose this design, which is really compact and ideal for righties and southpaws. It features an adjustable handle and a small strum thingy that is operated by a thumb. Yes it would be like holding a ukulele, but it would do the job.
It might be small and light enough to double as the singing microphone, for additional versatility. I have a compact drum design in mind as well, if this post proves to be popular.
Dave's Geeky Ideas 
Awesome!