I don't think there is a generic solution. It depends on the player, the DM, the PC, and the plot.
So giving me a bunch of generalities doesn't really let me give you a specific answer.
In general, any player who is committed to victory rather than fun is a potential problem, because it's impossible for the DM to promise that without illusionism which - if it is ever revealed - undermines the player's victory. In short, if a player has a goal he should never assume that the fun is going to be in reaching the goal, but rather in striving for the goal - even if his story ends in tragedy. A player who is 'ego gaming', that is he needs to have his choices validated as brilliant by their successful outcomes at all times, is quite often dysfunctional and heading toward some sort of great dissatisfaction and falling out with the DM, unless the DM is also the sort that doesn't mind being an illusionist and doesn't mind simply playing back to the player his own fantasies while simultaneously having the skill to pull that off without letting the player really know that everything is being fudged to let him succeed. The more ambitious the goals of the player, and the higher standard he's setting for success, the worse it is. If the PC's goal is, "I wish to become a global dictator.", can be satisfied by, "I am the Baron of Overhill, liege of 10,000 loyal citizens and that's pretty cool even if it isn't global overlord.", or can be satisfied by, "I'm conquered a good swathe of land before dying in battle, and will be known and remembered like a Naploean or an Alexander the Great of this world.", then you are good.
A DM on the other hand who is committed to a specific fore-story rather than simply constructing setting backstory is likewise a potential problem. One of the most serious crimes a DM can commit against himself and his players is spending a lot of time imagining specific scenes and how they will play out, since in doing so he is actually excluding the input of his players. Thus DMs that do that often find themselves at odds with their players and trying to force them to make the choices that they had imagined them making with such earnestness. This DMing crime manifests itself in all sorts of ways that don't look like stereotypical rail-roading - the DM that has every monster ambush the party for example probably is such a DM.
As secret backstory, campaign level secrets are essential to a fun and successful campaign. But they certainly shouldn't be secret PC backstories unless the player has agreed to let the DM mess with them. Many players of course really want the DM to mess with them and hard, but others - probably having been burned by prior bad experiences - don't, and some just really want to feel more in control of their own destiny. There is nothing wrong with that.
However a player in my opinion has no right to create a back story which makes demands on the entire cosmology of a world. That's why it's absolutely essential for the DM and player to negotiate a back story that both can be happy with. If the player's goal is literally impossible or even just extremely unlikely, the DM should be up front with the fact that he doesn't think the character can succeed even if he doesn't want to explain why. For example, if a deity in my game is known to be chaste, I'm never going to allow a PC backstory that involves the deity being their ancestor. Pick a different more fecund deity. And if the deity has a goal seducing a chaste deity I'm going to be up front about the fact that this cannot possibly end well. As a DM I'm pretty open about player driven world change, but even so some things represent ambitions that just aren't going to happen even if we played continuously for 20 years and the D20s were ever in your favor even when the odds weren't.