Even though it's still a ways off, here are some thoughts that I had while reading the thread:
I could see him doing a Guitar Hero: World Tour or Rock Band 2 polka, including songs that are on one of those games. It might seem a little too commercial, but then again he has done Polkamon, right? Call it Polka Hero, Polka Band, or something along those lines. I KNOW!! Guitar Hero: Weird Al Yankovic (now comes with an accordion peripheral!)... or I'm sure I'm not the first to suggest Accordion Hero, right...?
Depending on the strength and longevity, a few bands recently released albums: Alanis Morissette, Katy Perry, Coldplay, Gavin DeGraw, Asia?!? Def Leppard?!? lol Also think, how many bands have been included in both Parody and polka format? Not many. The only ones that come to mind are Green Day, Eminem, and maybe some that I'm overlooking. I'd like to hear from Fall Out Boy (Thnks fr th Mmrs etc), Coldplay (Viva la Vida etc), Daughtry (It's Not Over etc), The Fray (How to Save a Life etc), or maybe get a little into some techno polka. lol I'd also like to hear a They Might Be Giants style, polka, or parody, but to be honest, they are already pretty weird in their own right. That's what I like about them.
I also like that he sticks with the Pop, Rock, R&B, and Rap radio stations. That ensures that you get some of the best of what each type of station offers, while not getting most of the latest in Disney saccharine. These types of stations also regularly use some of the songs that came out within the last year, so we could still be hearing today's music. In any case, if a song has enough of an impact, or shows up in a movie around the time he starts on parodies, some of today's artists could become potential vict... I mean targe... I mean selections.
Maybe he could come up with something for the Watchmen movie.

If fear of failure keeps you from trying, then you have already failed.