Well, a natural focal point is enterprise. I could imagine (this is pure speculation) that there are a number of fairly cumbersome features holding back some large enterprise customers, but which has been de-prioritized in favor of the cloud product where gains are more immediate and viral. In general, there are probably many teams holding back on GitHub because of some missing feature which didn't make sense to build while they were bootstrapped.
And then, yes, there's expanding into adjacent verticals. They're already there with wikis and issue tracking, so it's not a virgin territory strategy.
I agree with your assessment. Deploying into enterprise takes a good chunk of money to deal with the countless "last mile" issues. Also, you start needing an enterprise sales team. Even if you use the Atlassian model, there's limited space for that now, because they were basically expanding into vacuum. I don't have a great sense of how big the available market is for "enterprise development management tools", but if Github wants to win there, it's going to have to put some marketing & sales resources into taking away mindshare from Atlassian.
And then, yes, there's expanding into adjacent verticals. They're already there with wikis and issue tracking, so it's not a virgin territory strategy.