Make a 64bit version of the game? Technically possible but unrealistic from marketing point of view: Cost a lot, gives nothing (in sales turms).
The only ONLY way I see Stardock doing 64bit version is that if game sells so well that they have really huge amount of extra money they did not planed on, users need it for extra huge maps and Brad think it would be a nice idea to play with.
Only if all those 3 condition met there is a chance for 64bit version.