Satellite Galaxies

How is this signifcantly different from just having a map with a lot more systems?

As you say, the satellite galaxy would need some reasonable game mechanism that allowed for travel to-and-from. But in effect, this is not really different from, as I say, having more systems as part of your starting conditions. Technically, it's simply a cluster of additional systems.

At best, you could *perhaps* describe the satellite galaxy as being "above" or "below" the main map. But I liken this to the notion of three-dimensional chess, where you have three boards, one atop the other, and you pieces can move between the different planes. This is novel to some extent, but does not really add to the game, and potentially is actually confusing to players.

As another consideration, I draw from David Brin's series of books in his "Uplift" series. In the backdrop of that series was the notion that the multitude of systems and alien races were originally composed of five galaxies. However, intergalactic trade between those galaxies was in decline because of stellar drift, where those galaxies were moving further apart, such that the usual travel time of years/decades between those galaxies was no longer feasible.