I've been following this thread and I'm finally going to chime in. No one (except the devs of course, and even they can't see the future) know how the new xpack will change gameplay. Bashing or praising it at this point in time is not going to do anything. It never hurts to be optimistic though. 
Also, Dark. Your definition of "Single Player" might be a little off. What you are referring to is a campaign which I know you've said before. But to say that the game has no SP just for the fact that it has no Campaign is not correct.
Campaign=SP
Skirmish=SP
SP does not always=Campaign
SP does not always=Skirmish
To clarify, you can have Campaign and/or Skirmish and it's still considered SP. One or the other, or both. I think that's one of the reasons everyone else is seeming to gang up on you. You fail to see the point that Skirmish is SP (just not the SP experience you want).
Onto the your point about the game not being designed for Diplomacy. If the game was not designed to have the option of diplomatic relations with other players, it would never have even been put into the original game. While the diplomacy system is very limited right now and pretty much broken, the xpack is supposed to flesh it out. And no one knows yet how the xpack will affect both SP and MP play. But saying that the game was not designed for diplomacy in any way is also not correct.
I personally believe that the game will always have a very RTS feel to it and that the 4X factors will take a backseat. Though the future is uncertain and who knows what this xpack will do. That doesn't mean, however, that the 4X goodies should be taken out. Everything put together is what makes Sins SINS.
I recommend that if you have such big problems with this game or just Diplomacy, forget it exists. Just play Vanilla or Entrenchment in Skirmish if you will. If you really need the Campaign option, you will have to go to another game. At least for now.
Thank you for taking the time to read this. I'm sure it's one of my longer-winded posts. 