When I made my first mashup video, I set out to make a music video of an actual mashup song, by mashing up the two videos from the songs in the mashup. The main criticism was that it was too long, and people didn't really get the concept. So I decided to redeem myself by making a mashup more along the lines of what the rest of the class did, finding a bunch of youtube clips with a common theme and mashing them up.
Basically, I found a bunch the weirdest bits of japanese culture that I could possibly find, and it turned out to be both very funny and very disturbing. Technically, this was a little harder than the last one because I was working with more videos more synchronization, but I think that I got the overall effect that I was going for. Let me know if there's any way that I can improve it.