I *just* got my N900, so some of these might already be possible / available in extras-devel (haven't been through there thoroughly yet), but here's my list:

1) A dead-fast way to get to the phone ... I had a couple of ideas on this:
a) Repurpose the unlock button to "unlock and go to phone" ... or make it configurable. The power button/swipe method is fine for me when I want to Internet.
b) Make an option to automatically go to the phone app when unlocking in portrait orientation.

... basically, some way to do it without any tapping on the touchscreen ...

2) I'd like to see a script that would automatically return the device to the desktop (but not close anything) when it locked for more than N minutes ... if it's been an hour since the device was last used, it's probably safe to assume that the next session is not a continuation of the previous one...

3) Option to auto-lock when closing the keyboard (and this is the one that I'm guessing already exists somewhere ... but after a lot of looking, I haven't found it).

4) scroll through recent calls & dial using hardware keys when in phone app. I propose using the volume keys to scroll, and the shutter release key to dial.

... so if anyone who knows the APIs well is looking for some hacking ideas, have at it

5. A calendar app widget (shortcut) that looks exactly like the default calendar app shortcut but shows the correct date. The default calendar app shortcut icon always shows date as 12. I don't like using Calendar widget because it is too big.

Does anyone know if it's possible to return to the desktop after closing an application, instead of going automatically to the task switcher (when there is another application running)? If it's not, I'd like to see that implemented in the future. I find that really annoying...