If anything, (I think) the contract creator should be able to set (1 "Must Do" condition) if they choose to. It shouldn't be an option for the creator every single time, only set it, if it makes sence with the mission (briefing story)
other than that, I also think the creator should be able to post (highly recommended 'suggestions.' Not that the player would HAVE to do what the creator said, just that the creator would strongly recommend it. But if the creator decides to post a special condition (for story purposes, which again, wouldn't be all the time) then I think the player would have to do it.
An example would be something like. "The client wants to send a message, by leaving a calling card, so make sure you place 1 "Merces Letifer" coin on the restaurant table."
Something like that would fit for a "story based contract" and doing something as little as that, wouldn't really interfere, or really constrict the player. But that's just my take on it man, to be honest I'm up for that, or just leaving it how it is now. Both are okay with me, but this is your thread. You decide what you see fit