And then, at least from what I got from one interview, the unlock system itself is, partially, to limit too much choice at once, to not get the player overwhelmed with what they would pick next. In particular when they're new to the game. If you go next age and has 11 options, it would take a long time analyzing every one, what each them do, and can lead to someone just picking any instead of evaluate the options they have. And that is at release, later on the options will just increase more and more.
On this part, I don't mind requirements being too similar. But I think they need to be more gameplay than thematic. For example, the idea of having settlements on the new continent unlocking America makes sense thematically, but don't think it is a good choice for unlock if it is doesn't work gameplay wise. If America is a civ that bonuses are focused on having cities in multiple continents and the like, then that would work, but if not then I think the unlock has to be something else.
The reason being, I think the unlock works better if they are well tuned with the bonuses the civ gives, so you naturally unlock civs that work well with what you have accomplished so far. Sadly the only example we have been revealed for gameplay unlock so far is Mongolia, and horses is something that would likely work with it both thematically and gameplay wise, so we don't know exactly if they will work on one way or the other.
So for example, if America has no bonuses about having cities in multiple continent I would rather the unlock have nothing to do with that. Instead, it be an unlock for lets says, Britain, if it has bonuses for that if the civ is based on Britain imperialism.