I think Thom1234 put it best what I'd personally like to see, though:
The ideal AD song for Rock Band (in my opinion,) would be long, progressive, challenging and technical, with lots of time signature changes, instrumental flourishes and soloing, while still retaining the great melodies and riffs in every other AD song we've seen on RBN.
It must have drum solos!!!
Kick ass drums, killer guitar riffs, and solos are all good things to have in a Rock Band song, of course. Personally, I think the most important things to making a fun song in Rock Band (speaking as someone who mainly plays drums and guitar) are technicality and variety. Having the guitar and drums both play a variety of technically complex riffs/patterns makes for an incredibly fun song. He Sleeps in a Grove is my favorite Amberian Dawn song to play of the ones that's out looks like it will provide some competition based on the preview video) because the drums and the guitar both switch patterns more often than the other two (and it has the longest guitar solo of the three).
Crazy drum fills and guitar licks popping up can also help. And, of course, solos can be great fun on both instruments (it's especially good if the other instruments get to do something cool while one is soloing, like how the drums get kind of crazy during He Sleeps in a Grove's guitar solo).
If you go through with this, I'm definitely looking forward to it. And also looking forward to the other stuff coming out, of course (Shallow Waters looks like it has a shot at taking He Sleeps in a Grove's spot for my favorite AD song to play in Rock Band).
How do you correctly pronounce the word "Tuoni"?
May 20th, 2012: 4 long years since HMX released "Hysteria" by Muse. The wait for more Muse DLC has to end at some point, right?
Perfect RBN song, have solos for every instrument! Drums, Guitar, Bass, and VOX solos would be sweet