If you didn't have much in mind for the greater campaign arc, there's nothing wrong with letting those goals guide the sessions if it interests you too, but as MC indicates starting/operating a gang/cartel isn't really Running. No doubt the skill-sets of Runners could be handy in doing the "exciting" parts of it (taking out rival leaders, sabotaging labs, smuggling loads, fighting over territory, etc.), but in reality the people who run the gang don't do those things - that's the mooks who become cannon-fodder for Runners. It's largely manipulating and managing people - and make no mistake you'd probably have to have a lot of people who believe in and trust you to successfully build a gang from the ground up. Also, make no mistake, you shouldn't feel obligated to let their goals dominate your story if you had one that you were excited about and wanted to tell that doesn't mesh with this aspiration.
But maybe that last part is just my view because I game with close friends who respect each GMs turn at the helm as our buddy's chance to tell a story they've probably been working on for quite some time, and even value that GM's vision and want to see it manifest, maybe that's not your situation?
And don't get me wrong, there should be room in a campaign for players to explore character aspirations, but if you envision Runners jetting to Berlin and Tokyo and what not, players who are obsessed with fighting over blocks in the Sprawl is a pretty big conflict. If they're building their PCs around this concept, and you're not 100% on board, definitely tell them that this is not the kind of campaign you're running. If you play frequently enough that there's room for that substantial of a subplot, that's great they can work on those goals in their down-time, but really you're the one that has to put time and effort into planning all this stuff, it's okay to say no.
