You could use the extra space to display additional information about a player's progress. Obviously this couldn't be essential information otherwise those using 4:3 monitors would be playing at a disadvantage (unless they had a way of accessing it during play). Also, the information that you already display could be moved into this area.
Also, you could consider displaying more of the play area to widescreen users. Although the advantage over 4:3 users is here again, in which case you could allow 4:3 players to play the game in widescreen format, i.e. with bars at the top and bottom of the screen.
Also, you could consider displaying more of the play area to widescreen users. Although the advantage over 4:3 users is here again, in which case you could allow 4:3 players to play the game in widescreen format, i.e. with bars at the top and bottom of the screen.