Interesting case, but as it usually happens, the only fair answer to most of interest questions, is that it all depends. Finding own balance and striving for balance in general is not something that can be automatized, but it certainly is good to have some guidelines to follow.
Personally, I would recommend looking at the "Ockhams Razor" principle. In the evolved meaning, it says the amount of entities should be kept down to minimum, to a degree where each entity serves a unique purpose. Therefore, if certain idea does not fit in one game or the implementation of which the idea would disrupt the intended cohesion of one game, it is better to be put the concept into another game, but then, as soon as we have two games to work upon simultaneously, each arising idea has to be directed either to the first or the second game, which already could pose a problem to decide. In general, the more the things, the more the distraction, but I do not believe everything in the world either could or should be put into a single game, because games, are not meant to emulate the world, as they are worlds themselves, of fewer dimensions.