I think that there are 4 sorts of improvement ideas:
1. Nokia hardware
2. Other hardware
3. Maemo/Nokia software
4. Other software

1. Anything related to Nokia Hardware:
built-in Infrared, bigger screen, other connectors/storage media...
Basically that's an idea for the next Maemo-compatible device, son-of-770. Difficult to participate from the ouside.

2. Anything related to other hardware:
a bluetooth-to-infrared converter, an adjustable stand, an ebony case inlayed with mother-of-pearl "don't panic", extended battery, bluetooth headset connection...
So a third-party, commercial provider steps in , like what happened to I-pod speakers and stands. Or is already there : bluetooth headsets. There may be a compatibility issue when it comes to drivers, to be solved either by Nokia, or by an open source partner, or by a good hacker (who volunteers to write the bluetooth headset driver?).

3. Anything related to basic, internal Maemo/Nokia software.
Just to add my my own: I want to have the strength of the WIFI connection shown as 1 to 4 bars in the icon. I really don't understand why this is shown for the light intensity, the speaker volume, the battery, but not for the WIFI signal strength? In particular since they re-designed the icon for OS2006. It's more beautiful now, but nor more useful.
These things are best kept on a wishlist indended to be read by decision-makers at Nokia. Yes, someone else might write a small program or extension, but that kind of features should go to the core.

4. Anything related to other software that can be provided - in principle - by anyone who is just good at writing code.
Like my own wish: a multi-source, automatic localization platform, taking the info either from GPS (see Maemo Mapper), or from the known location of the phone connected via bluetooth, or from the best guess location of the wireless hotspot, or from manual input. And then sending this info to any other program that might use it.
This is the "usual" scenario. Limited by reources of people willing to give away their expertise for free, or trying to generate some income from open source / shareware / beggarware, or service.