To me, the similar single-player rules are a little off-putting because of the way some of my current co-op games play out, namely Pandemic and Elder Signs.
The handful of times I've played the aforementioned games, it's been with different people every session, and as a result of being the person with the most experience playing these games, more often than not I have my friends just looking to me to see what they should do, at which point I feel like I may as well be playing by myself. Maybe it's just an issue of having inexperienced players and I need to give it more time. Maybe things will change once my friends have become more accustomed to the rules. Otherwise, I'm probably just looking for something that doesn't exist and this kind of situation is just inherent to co-op games in general.
Thank you for your viewpoint. My co-op games are often played with different people as well. So when new players look at me for what to do, I usually just say something like "You should do whatever you think is best." along with "It's just a game, feel free to explore and have fun." Usually after 2-3 iterations of this, they stop looking at me. But in my experience, these games are the ones they often clamor for me to bring out again.
Then, I control my character as best as I can given their actions, which provides an additional level of difficulty and excitement when playing with new players. That being said, I agree that co-op games are best when people are on similar levels of experience and people can have a good debate about best path to follow. It is interesting that games designed to be free from the traditional "winners/losers" post-game bitterness end up with players feeling embarrassed about making silly moves.
Does anyone have any examples of boardgames where there are well-structured rules but players can construct their own (personal) "victory" condition? I don't mean secret objectives per se, more like a game without victory points in which each person writes down their own victory condition. These conditions may be mutually exclusive or entirely cooperative, or anywhere in between. At the end, everyone reveals their condition and whether they succeeded. Or maybe the conditions are revealed halfway through the game for additional hilarity. Examples from Catan would be like "End with the longest road" or "Build four cities" or "Produce the most wood and sheep" or "Give away 100 goods via trade".
When there is no set victory condition, I think embarrassment would be reduced, and the game could still be co-operative depending on the group. I suppose the downside is the lack of playtesting -- it may take several plays before people have a sense of what kind of victory is feasible or interesting.
Edit: Perhaps one practical compromise would be a game with a spread of victory cards. Each victory card has something nifty like the ones above it. Players go around selecting victory cards from the full spread until either everyone is satisfied or cards are gone. Perhaps these victory cards are also translated into victory points. This would be somewhat akin to ticket to ride, if you laid all the train tickets out from the start. But instead of very similar cards (route between Philadelphia and New York, between Pittsburg and New York), you could end up with some interesting cards. I kind of like the original, unconstrained version of the idea above, however.
but soon you will have expansions and a Dominion sized box at minimum will be necessary.