I try to keep this list somewhat prioritized. The ones that will give the most bang for my coding buck are usually at the top.

Short term

1. Add a way to filter out certain categories of players from the Who View window.  (Requested by DBA.)  Could have a menu with checkbox items for Open, Looking, etc..   More comprehensive would be to add widgets to specify a rank range, min idle time, etc.
2. Add a button to the game window which does two things:
1. Gives visual feedback as to whether the user is browsing.
2. When clicked, either starts browsing or returns to the game-in-progress.
Currently it can be very hard to get back to the game-in-progress if you start browsing while at the end of the game tree.
3. Add a "tall" configuration to the game windows. This makes it easier to watch a game on the side while you're mainly working in some other window. (Requested by dfan and myself.)
4. Add realistic stone-click sound.
5. Add some visual feedback to the terminal window for whether the user is open/closed/looking.
6. Issue a "games n" command when a match is started, in order to keep the games window up to date. Requested by geek.
7. Add an audible warning when time is low. Requested by tt. Should allow user to specify a list of times (in the init file) at which the sound will be made.
8. Add "Copy as ASCII", "Paste SGF", and "Paste ASCII" (in that order of priority) to the popup menu on game boards. This way I can cut some SGF from a mail or news message and paste it directly into a game, and don't have to save it as a file first.
9. It would be great to have some SGF files distributed with Ergo, such as some life and death problems.
10. Make wood-grain board brightness settable.
11. Implement a "notify" command. They say that WinIGC calls this the "Friends" feature. Basically a list of people you want to notice when they connect. This could also be done with the scripting feature described below.
12. Add support for entering move sequences e.g., right-click on a variation and choose "Copy" to copy the sequence of moves to the clipboard as "b k2, w l2, m3, j4 ..." string. And, hold down the Alt key and click on the board to insert a given position into the input field.
13. Ability to mail SGF record of the game. Useful when run as an applet.
14. Better "game over" display, or even a dialog saying "The game has ended. White won by resign." or whatever. Requested by geek.
15. Add buttons to toggle open/closed, looking, etc. Requested by geek. These are especially useful since they also display the current values.
16. Display the current value of dot (.). Perhaps to the right of the input line.
17. Add a display of the game tree so the user can see where they are as they browse, and select variations from the display.  Can use a tree-control, a la winigc.
18. Context-sensitive help.
19. Better general help.  Perhaps just make the URL in the About Ergo dialog be clickable.
20. Use different colors in the Who View window to show whether someone is looking/open/closed. (Requested by dfan.) It occurs to me that this isn't hugely important because you can always use "who all o" etc to get this info quickly.
21. Add ability to clone a window.  (Requested by study and landon.)  This allows you to compare the results of two variations side by side or to really explore a position while still paying some attention to the game in progress.
22. Add "Undo to position" to the right-click popup menu on the game window.  (Requested by study.)
23. Add some UI to support options. A dialog box that allows the user to change fonts, colors, sound, servers, etc. For now you have to edit the Ergo init file.
24. Display a temporary move (visually distinct from a normal move) where the user clicks, until the actual move is received from the server.  This gives the user feedback that Ergo did the right thing and they're just waiting for the server now.
25. Add "Show Observers" and other menu items to the game window. It's useful to have these on Windows 95/NT, where you can't type in a window unless it's on top. This means a lot of clicking back and forth between windows, and having menu items for some things may reduce that. So would having a full-screen mode where a single game is attached to the main window...
26. Allow user to specify a directory where games they participate in will automatically be saved.  The date and time should be appended to the filename to keep them separate.
27. Implement "undo n" in Ergo for servers that don't support it (i.e. IGS).
28. If the user choses File > Close during an active game Ergo should prompt for one of Cancel, Adjourn, Resign, Done Scoring, or Do It Anyway.  (or something like that)
29. Allow an input prefix string (e.g., the user could set it to "tell geek") that would be added to all outgoing messages.  I always hated this feature because it's so prone to mistakes, but I think I know a way to implement it that will prevent that.  Simply insert the prefix into the input buffer immediately after the user presses Enter.  That way it will be so obvious you can't miss it. (?)
30. When Ergo notices a huge territory with only a few enemy stones in it, it should be extremely suspicuous about these stones (the user probably forgot to remove them) and ask the players about their status. (Suggested by JanSteen (on r.g.g?).)

Long term

1. Improve scoring mode by allowing the user to "paint" the territory boundaries. They can drag the mouse to draw grey stones to finish off territories. The scoring algorithm will consider a grey stone to be neither black nor white.
2. Improve the SGF reader. It is currently pretty weak.
3. Support Ishi format (requested by Carlo Tibaldi)
4. Internationalize it. Provide Japanese, Chinese, etc. menus and commands.
5. Provide some basic scriptability. Allow the user to do basic macros. i.e., "if you see '%s has connected' send 'tell %s hi'" or 'popup "%s has connected"'. Actually, I'm not sure I see much use in this command. I never want to say anything without thinking.