Ah yes, this "second galaxy" suggestion comes up from time to time, and is always quickly met with "but we've only discovered 0.04% of all systems". Thing is, people often suggest this because the galaxy on a macro level is pretty much explored, not on the system level: there are no areas, regions where nobody or only a few have been, there are no undiscovered nebulae (save for a very few procedurally generated small planetary ones) and so on. They'd like a new galaxy to be able to go where few have gone.
Trouble is, as long as the new galaxy's gameplay mechanics stay the same - everything visible on the galaxy map, no danger, easy travel and so on - the "era of discovery" would last a month or maybe two, seeing that everyone would be crowding to rush out into that new one. After that, we'd be back to where we started from. Then soon enough, you'd get some new suggestions: add a third galaxy please...
Although personally, even without new mechanics, I would like to see another (dwarf?) galaxy with some of the Forge's errors fixed, and no fleet carriers. One small inhabited area (the entry point) and nothing else. This would be like the "original" galaxy of ED, when you could only dock in the bubble, and nowhere else.
Of course, I'd love to see a more dangerous and more unknown galaxy, but to create that would require much more effort.