I ended up going for my second idea, of the city skyline and a track down into the underground music scene.
This involved both 3D and 2D elements and a lot of particles generated using the AFX plugin Particular.
I set up a 3D comp in AFX and spent too long making nice smoke plumes for the skyline, a skyline which I had drawn in ink and then scanned in. I then went on to create a flock of birds in the sky using particles where each particle was its own comp of a bird doing a simple up down wing flap animation, this worked suprisingly well.

By now I had hit a dead end as to how the tunnels under my city were actually going to appear but in the end, I drew the tunnels in Photoshop, and then lined the up in the comp, then made an enormous mask that could reveal them as the camera tracked down. Really simple and not as elaborate as what I originally had in mind but effective. I then spent a while making sure the mask and camera move timing were really tight together so that the red room would be revealed just as the camera reached the correct depth.

This then brought me to the cassette. I had looked at some reference (the ever useful google) and picked a suitably retro looking tape to base my model on. I ended up using this reference as the texture too seeing as it would already fit the model-theoretically. Thanks to some stupid modelling discipline on my part: smooth shading it beyond reason, the UVs were a bit of a nightmare as always. Anyway, I modelled it, applied the tecxture, tweaked the UVs, made specular transparency and bump maps in Photoshop. This greatly enhanced the realism of the way light fell across the model. As always click to enlarge:

And one with motion blur ;)

This took quite a while but it was worth it for the chance to add a bit of dimensionality and hone my modelling technique, despite the tapes bit part in the ident as a whole.

Here is the Bearded Man from the first idea which discarded because he looks like crap, apart from the awesome beard, I just couldnt get the look I was going for. Wierd, the human form eludes me yet again, no matter which medium I try my hand at.

 I then went on to add some dust to add the sense of descent as the was nothing but the tunnels to convey the camera's movement down the Y axis. Having reached the denouement of the ident I was unsure how end it. After much procrastination I settled on a wonky camera angle and the big shiny words appearing as the music began to play after cassette clicked into place in the red room. A lot of this was reliant on sound so I rendered the comp and imported into Pro Tools which worked extremely well, it had no problem playing the video alongside whatever sound recordings I downloaded from freesound.org. I then placed some of my own music underneath these effects as if it were underground and then had it play in all its glory as the tape clicked into place. Copyright infringment cunningly avoided. Here is the final 1080p render: