I am taking ideas for things to clone, reverse engineer or figure out the inner workings of.

Things I have already done (and am continuing to work on):
Development package for the closed-source browser-neteal library (used by the web browser, conversations app and tutorial/getting started app to talk to the browser daemon)
Clone of the tablet-browser-view-test package as an example of how to use tablet-browser-view-dev and my browser-neteal-dev package to embed a "browser view" (which takes care of things like zooming and scrolling for you)
Partial development package for the closed libconnui and libconnui_cell libraries used by various connectivity widgets (control panels, status bar widgets, connectivity dialogs etc) to handle things like cell modem status, airplane mode, cell system state (operator you are connected to, cell tower you are connected to, 2G vs 3G etc) and some other stuff.
Custom operator widget which supports display of Cell Tower IDs sent by cell towers as Cell Broadcast SMS messages on channel 50.
Partial header file containing information related to the stock lockscreen and how it works (i.e. information that would be required by people who want to clone the stock lockscreen)
Functionally-identical copy (based on older source code) of the browserd daemon (which will benefit people who want to enhance the MicroB browser or fix bugs in the rendering engine or possibly even bring in a newer Gecko version.

Things I will not do at this point:
Anything related to the kernel, hardware drivers/firmware, cell modem firmware, X11, graphics system or bootloader.
Anything not concerning Maemo5 Fremantle on the N900 (i.e. anything related to the N9, N950 or to MeeGo)
Anything related to the Cellular Services Daemon and talking to the Cell Modem (documenting how to talk to the Cellular Services Daemon over dbus I may do as part of other work I end up doing but I will not be doing work specifically on the Cellular Services Daemon)
Anything related to location or GPS (e.g. figuring out how GPS related logic works)
Anything related to cloning the browser UI (I will take requests like "can you find out how the browser does xyz" but I will not take requests to clone any or all of the browser UI)
Anything related to intellisync, calendar, clock, alarm, messaging, chat, calls, voice chat, video chat, email or camera, media player, photos, calculator, OVI store, My Nokia, Amazon, Facebook, AP News, ForceaWeather, Google, notes, app manager, PDF reader, File manager, RSS, Sketch, terminal, blocks, chess, mahjong, marbles, sharing, bluetooth, sync, FM (transmitter or receiver), or DocsToGo.
Anything related to BME
Anything related to stuff not pre-installed on the phone (e.g. stuff from extras)
Anything related to packages that are open source or that have already been copied by the community.
Any requests related to the virtual keyboard other than feature-identical clones of existing closed source binaries and dev packages for existing closed source libraries.

Things I am considering:
Investigating the Nokia Maps HTML and browser plugins to figure out if it is possible to add display of Google Transit GTFS data (which is made available by a number of public transit operators including my local operator)
Figure out the encrypted backup format used by osso-backup to allow you to restore backups without a N900.
A feature-identical clone of the libhildon-im-vkbrenderer library which is used to render the virtual keyboard (including the special "symbols" keyboard you get when you press the "sym" key on the hardware keyboard).
Feature-identical clones of other libraries related to the virtual keyboard
Further clones of the connectivity widgets, control panels and related middleware/dialogs (requests are being taken for things I should look into here)
A clone of the stock swipe-to-unlock screen (or partial clone,i.e. not doing the clock/time stuff or the "unanswered messages" stuff)

If you have any specific requests (including requests connected to the above todo list), post here and I will respond telling you whether its something I will look into or not.

I'm not sure it fits within your list of possibilities, but one amazing improvement to the N900 at this stage would be cloning the closed source Opera virtual keyboard to substitute the stock virtual keyboard.

To me the current virtual keyboard is a true weakness of Maemo, especially when using it portrait with CSSU.

I already said that I wont be working with software that doesn't come with the N900.

Can you please tell me if I can change the letters "ABC" "DEF" ... that appear on the right of the screen, when selecting a contact while on portrait mode? I need to use greek letters because my contacts are in greek.

That'll be awesome

- Better portrait mode with photos.
- Portrair mode calendar.
- Portrait mode in clock and alarms.

jonwil, hope you find much fun and enjoyment tinkering on N900.

While you stated these are beyond your scope of interest, I'd love to see someone take up OTR for telepathy, SSL for phone calls, or finishing tinymail's PGP/SMIME and making it available in Modest.

browserd supporting new versions of Gecko would be nice for security. Another potential perk is if new Gecko builds happen to support iPhone or Android Javascript touch events (touchstart and friends). This would make N900 a viable portable development platform for modern touch web-applications.

Count me in, if you need help for the clock/time and "unanswered messages" stuff.

And I am interested in an open source clone
of nokias telepathy stuff for the messaging-ui
(package rtcom-messaging-ui)

As am I, I was discussing this on #maemo the other day actually, where jonwil did a initial bit of investigation that shows the dependencies aren't too bad at all. It's a few projects down my todo list, but it's on there. Feel free to do the heavy lifting before I get there though

Did you read post #1 at all?

camera front
android
potrait keyboard