So long as it's a .png then it doesn't really matter, but png8 doesn't have enough colour depth for more complex images and png32 isn't required as we don't need an alpha channel for these. (The only reason I want them as .pngs is to keep them all the same as I may need to use them with some dynamic output at some point and it makes life easier)
Can certainly do coloured usernames if you like.
Not sure about a to-do list as it sounds like a bit of a redundant feature.
Changing fonts is fraught with potential issues. It would be possible to allow people to use different fonts for their forum posts (via bbcode tags for instance) where it wouldn't affect layout as the text wraps anyway (changing to verdana site-wide would mess up a lot of layout because it has wider kerning for example), but if you're thinking of letting people use crazy weird fonts for a laugh then that doesn't work because you have to stick to what fonts people have installed on their system. There are also rendering and display issues with certain fonts, especially cross-application and cross-platform.
This is why all websites use either Arial or Verdana with a fallback to sans-serif (there are about a dozen web-safe fonts we're restricted to).
Being able to choose between Arial and Calibri isn't exactly the most interesting feature ever
There are javascript based scripts available that will render non-standard fonts without the end user needing it to be installed, but they all have issues with special characters, hyperlinks and other issues. You can also use php to generate the text as an image, but that doesn't work for smileys/links etc and is basically hell to parse correctly for the image output.
You can embed the fonts via css, which would download any custom font when you view the page. The problem with this, again, is support. IE supports it but only in MS's proprietary EOT format. The valid CSS3 technique only supports TTF (truetype font) . This @font-face property is now supported by Firefox 3.5+, Opera and Safari, but isn't enabled in Chrome by default - it's a command line option (--enable-remote-font). Opera is also very picky about the syntax used and the code can break easily.
There is also the additional overhead of the font download to consider.
All in all it's a rather troublesome problem!
I could add some bbcode tags for some custom fonts and just use the @font-face CSS3 method since it degrades gracefully, and hard cheese to IE users! It would spice up forum posts I suppose!
Ok I've argued myself into taking a look at it
Yes I think aaron's ideas are great, although they'd be more of a feature than an unlockable.