I don't think a potential acquisition would mean much for Heroku's independence, they continued business as usual after the SFDC acquisition, so it'd make no sense for Microsoft to worry about rolling them into Azure (these things take years in practice), at least in the short term.
Microsoft is definitely a lot friendlier to Linux and open source these days, though I think a Microsoft-owned hosting platform atop Linux and AWS would be .. strange. I suspect they might even divest it as part of the acquisition.
They do support many more platforms, though I suspect that's still the lion's share of their deployments, they're still very active in that community, and they still support Ruby development through Matz's paycheck. On the flip side, Heroku is still a major deployment option amongst many startup and shops that rely on Rails. If anything happened to Heroku (I doubt it would) it would have a very significant impact on the Rails world.
