But their are absolutely no restrictions for playing only with two players? Same game rules as for three players? I think it is possible with lesser players besides of being "funnier" with three and more.
You could play it that way if you want, although it would boring as all get out. In addition to having no use for alliances, a significant number of faction abilities become trivially predictable if there are only two players. Odds are at least one of you would be stuck in a no-win scenario, or else fighting tooth and nail merely to avoid imminent defeat.
- My opponent is Jol-Nar. He knows most, if not all, of the cards in my hand because he can peek at them before I buy them. Remembering what I have can't be too hard when I'm the only other player. Plus he can peek at one of the things I'm using in each battle. It would be very difficult for me to surprise him in combat, which gives him a powerful advantage throughout the game.
- My opponent is Letnev. He either has a mitt full of my traitors, in which case I won't be winning any strategically important battles this game, or he has a mitt full of his own traitors, in which case one of his most powerful faction abilities is neutered. One of us will be hating this game from the word go based solely on the luck of the draw.
- My opponent is Lazax. If I want to buy cards I have to give all my money to my only opponent, which means he gets spend all of the influence he can gather plus a healthy chunk of what I can gather to rebuild his losses from any battles we may fight. He can also easily bid up the price on cards because he knows how desperate I'll be to have them, and with only one opponent to deal with, he needn't worry about blowing all his cash on accidental wins.
- My opponent is Hacan. I can't control strongholds without landing troops, and I can't land troops without paying my opponent. My opponent can focus on slaughtering my troops and make a profit on my efforts to stay in the game. To make matters worse, he wins if time runs out, so he has no reason to go for strongholds himself. (Unless I am Sol, in which case I'm laughing and Hacan struggles to stalemate the game without letting me meet my own special victory conditions.)
- My opponent is Sol. He can land troops for free, he's immune to offensives and can predict the bombardment, so the only thing he fears losing troops to is me. On the up side, his special victory is easy enough to block. Of all the possible opponents I might face, this is the least depressing. And, of course, if my opponent is Sol that means that HE is facing one of the other five scenarios described here, so even if I'm happy with this situation, my opponent likely isn't.
- My opponent is Xxcha. Gee, I wonder which faction he'll predict for victory? The only variable to consider is what round I might win on, and even then, predicting turn 8 and then doing everything in his power to delay me isn't a difficult strategy. His ability to coexist limits my ability to strike back at him or make pre-emptive attacks.
I've said this before and I'll say it again: the thing that makes this game such a gem is the interaction between players. Wheeling, dealing, bluffing and flat out lying. The mechanics are good, but they're nothing special. This game truly is the more, the merrier.