At the end of the day, use the N810, and develop ideas and software to fill in the usage gaps you have, because chances are others will have the same gaps.
Check to make sure they have not been covered before though. Apps present on N900 but not N8x0 are very welcome to fill in the gaps.

One idea which I would like to see is an applet (similar to the Internet Radio applet) to shuffle play the media library.

I'm not into gaming myself, but the newly released (and currently very raw) are the new 3D accelerated drivers. I think a big area for usage of these drivers could be in games on the N8x0.