I agree that its not a problem to have code supporting more playable countries than are actually available to the player, so a mod doesn't have to provide more countries and the patch won't break the single player campaigns (you can't add extra ones to the end of the list either BTW).
On the topic of a whole new side, that requires extensive additions to give them the same features as the other 3 sides (and even WW didn't include all of those for Yuri, though that is mainly an asset issue). My opinion is that all the other unit based logic should be completed first and then worry about a 4th side...then you might actually be able to make a few unique units to fill that side and give them unique super weapons rather than just cobble together something. That and the fact that although I've seen plenty of moders try and add a forth side of sorts, I've never seen any good ideas for actually using one.
On the topic of a whole new side, that requires extensive additions to give them the same features as the other 3 sides (and even WW didn't include all of those for Yuri, though that is mainly an asset issue). My opinion is that all the other unit based logic should be completed first and then worry about a 4th side...then you might actually be able to make a few unique units to fill that side and give them unique super weapons rather than just cobble together something. That and the fact that although I've seen plenty of moders try and add a forth side of sorts, I've never seen any good ideas for actually using one.