I like the way it presents in Falout 4 Far Harbor. We found new colony, we could subjugate everyone (Institute for the win!!!), we could kill everyone or we could help them out. That DLC has a lot of choices, we could convince everyone to kill themselves even. I want more of those choices in the game, the practical evil choices. Bioware lacks evil options the more games they have. DAO, we could lead the werewolves back to slaughter everyone or side with the golem, in DA2 and DAI, it's all about saving one or the other (like between mages or templars), but they're all essentially good.
I agree about Fall Harbor. Having just finished it last night, Far Harbor was a refreshing take on what was effectively a land-conflict. It's mixed, ambiguous, and far from one-sided. It might actually be too subtle for some people, considering what the internet is calling the 'good' option, but it doesn't beat you about the head with a morality stick either. Add on that a plethora of end-states for the DLC, and it's probably the best RPG content in Fallout 4 bar none.
As for Bioware and evil, though... honestly, I'm glad they've gone away from the 'evil' choice options, because quite frankly Bioware was just plain bad at writing evil. Mass Effect had the problem of Paragon was always right, but even before then evil was hard to justify on it's own terms- it was generally short-sighted, pointlessly cruel, and objectively inferior to the rewards of a 'good' route. It was also often boring- lacking the follow-up, the reactions, or the nuance of less totally-evil actions. In the Dwarven arc, there was no 'good' King and the Anvil of the Void had a serious moral delimma behind it. By comparison, the werewolves pretty much had to be badgered into something they didn't want, in a dialogue chain that renounces elven support (the reason you're there) even before you can suggest the werewolves as an alternative. And that doesn't even touch on the 'evil' elements where the characters involved are complete caricatures.
In all honesty, I think DAI had the best moral choices/Big Decisions of the series. They lost the 'evil,' sure, but they also did away with the 'this is overwhelmingly good' route and so you had a lot more flexibility for moral arguments.
Why would we invade everyone instead of finding a new planet? It's the same reason why bandits attack other villages, those places are already stable, forming, and have a lot of food, it removes us having to do it ourselves which may take hundreds of years. It's all about choices after all. For a game with choices, you can't just put good choices in the game. Yes, we should handle it peacefully for those who want peace among everyone, but for others who may want to skip all the hassle of spending hundreds of years making a place hospitable, invasion should be an option for them.
And this is why the OP's angle is probably not going to happen. There is an answer to this, mind you- conquest can allow looting (quick and easy resources), taking over established infrastructure (exploiting capital), and exploiting the population as a labor force that you wouldn't have if you just colonized empty space yourself (bodies to use). It also can remove future competitors.
But that's (probably) not how we're going to go into the galaxy. We're probably going to be having disputes over 'terra nullis'- claimed but not-really-settled regions as we try to set up shop somewhere that we won't get killed. Proctorates are possible- but which side we're on is up for debate. Conquistadores, though- probably not.
Knowing Bioware, it's probably just gonna be able helping everyone out yet again.
Trading favors and building an alliances to get a colony world, at least.
We know there's the evil-alien faction. It just remains to see what sort of polity or alliance opposes them.