So, most people think that the first things to do are getting rid of limits and well-known bugs.
With apologies to everyone who contributed.
Remove the 200 sprite limit (simply put, CtP2 can only use 200 sprites).
Fix inifinite rush-buy bug for MP (this one's evil) [Martin believes rush-buy button should be disabled when it's not your turn]
Fix scenario loding bug (what's that?)
Fix sea city sprites (you know when you look at them)
Fix PBEM, this including adding the human-to-human diplomacy. Already done partially due to mods, so should not be too complex.
[I believe that fixing PBEM is of great importance, that could attract many more to CtP2 realm]
Fix the bug that lets sea units beenfit from undersea tunnels
Also, some general AI improvements have been suggested, but those, IMHO, can wait till we get the worst of the bugs fixed.
Add interface support for games with more than 8 opponents (that is, so that you can start those without editing text files).
Of course, bring the PBEM and hotseat buttons back on interface by default when those modes work.
Fix the bug where clicking the eye in a dialog box centers on city, but doesn't select it.
Arguable: replace the message tab with something like message icons from CtP1
Arguable: replace Mod-swapper with something in-game. Not a bad idea, but not top priority, IMHO, as the Modswapper works, too.
Arguable: Add ruins to easiest difficulties. Or at least make that an option.
Allow units disbanded from the "unit" screen in city display to contribute to production.
In city manager unit screen, add an indicator for fortified units.
Increase GL "lemur poo" chance from 1% to 95% .
Add interface support for changing player color.
Fix the freeze bug at goody huts at units or advances.