Who's to say that in the 20 YEARS Obi-Wan watched over Luke, that Obi-Wan didn't fight off potential attackers/assassins/kidnappers? For example, maybe someone in the Empire, beneath Vader, happens upon some sensitive information, and puts the pieces together that there's potentially another Skywalker on the very same planet - in the very same city - that Anakin was raised. So he takes a small contingent to Tatooine to confirm, and once he does, attempts to kidnap Luke to take him back to Vader, possibly in attempt to win Vader's favor. But Obi-Wan finds out and somehow thwarts the attempt. All while having to deal with and intervene in some kind of potential civil war or uprising on Tatooine over water scarcity or something. While ALSO trying to remain as inconspicuous as possible. The possibilities are nearly endless as to what Obi-Wan could have to deal with while watching over and protecting Luke, almost as some kind of invisible guardian angel of sorts. It could be an AMAZING story. Think Lawrence of Arabia meets The Professional meets Star Wars, except the kid can never know he has a protector.