Probably very hard but it depends on the data that is used and how advanced the tools are that the advertisers use to set their parameters. From what I can see we have lots of text for keywords, tags, reputation, views, upvotes, downvotes, post date, and probably even more. All of this kind of thing can be used by the advertiser to narrow down to their audience they think would buy their thing. Beyond that it's about as simple as adding some banner ads on the most popular steem sites, and even though nothing is ever as simple as it sounds conceptually (most likely a ton of problems to solve just to have banners loading the correct ads) in the end it would be a multiplier to the value of steem, so I think it's worth as much effort as anything really.