I don't have this expansion yet, but I've read through the rules for it, and I really liked the idea of the Treaty Cards. Though, it seems that since they are only used for the Fall of the Empire scenerio that there are only cards for 7 players (one of which is the Lazax) and that the others are for the specific races that take part in the scenerio. I would have loved to see this option added to the normal game play. It seems like a cool way to add some more intrigue to the game, not knowing who has aligned themeselves with whom.

Does anyone have any thoughts on this (good idea vs bad idea), and does anyone know if there are special rules on the cards other than the numbers? Could I just make my own up for each race so that I can add this aspect into my next game?

If you want to add more intrigue I suggest to use either text messages or little pieces of paper. If you use text messages you can make it almost impossible for other people to know with whom you are allied. And if you are discrete while handing over the paper you have the same result. I think that is even better than adding the Treaty Cards since you can write whatever you want, you don't have to stick to the text of the cards.

But of course you can add your own rules, just make sure that everyone understood it, nothing is worse than people explaining about the new rules in the middle of the game because they didn't say anything while you explained it.