First, I know the Prequels are huge targets of opportunity. There are so many things one could fix! And think of all the lives one could save by making Anakin not turn to the Dark Side! BUT. It would be awesome to see some Original Trilogy AUs, or more AUs that involved Han, Luke, and Leia, because dammit I LOVE THEM. And if you could get Mara and Lando involved too, that would be great. I can think of several AUs that would be awesome to read. Or how about, instead of Obi-Wan time-traveling back, you have the OT group somehow get sent to a parallel universe where Anakin never turned, and things are way different, and they have to figure out how to get back to their own universe. Something.
Second. If you are going to write something where Obi-Wan goes back in time to his younger self (there are several where his consciousness from the OT era ends up in his PT era body), please do not make him magically an idiot. He's back in time to change things, right? Instead of making up reasons why he can only make small changes for blatantly plot reasons, have him do something really unexpected that changes things so that Palpatine is left going, wait, what? and scrambling madly to catch up. All of those reasons why (for one reason or another) things have to happen pretty closely to the way they did in the movies just come across as shoddy writing. For a particularly egregious example, there is a story where he arrived back in time just before AotC started, and he was going to try to prevent the Battle of Geonosis from starting by capturing Jango Fett so he never had to go to Geonosis, but he failed! so he went to Geonosis anyway. I'm like, Dude. You know where he's going and what you're going to find there. Send the information to the Council (and maybe other information you know because you are a Time Traveler,) and capture him later! Or, if you have to go to Geonosis, don't eavesdrop on the SAME MEETING YOU DID BEFORE. Break into Dooku's office for proof he's a Sith or something!
Third. I get that Obi-Wan and Anakin have a close relationship. But if you're having Obi-Wan go back in time from Vader-era to Anakin-era, I'm pretty sure it's not going to be as easy as "oh, sob, he did horrible things, but let's forget about all of that because he's young and cute now and if I let myself love him maybe he won't turn!" Make them work for it! It would make for a much better story, and far more interesting. Anakin murdered all of Obi-Wan's friends. Personally. And then went off and did horrible things to the rest of the galaxy. It may not have happened yet in this galaxy, but in this scenario Obi-Wan remembers it happening, and has to have at least some doubts that it may happen again. Don't just sweep that under the rug!
Fourth. If you are going to have Anakin be the one to have his consciousness thrown back in time ... let us not make him completely unrecognizable as a character, mmmkay? Like, this is a dude with a wicked temper and a taste for killing helpless people he didn't like when he was a good guy. Instead of him trying to be the Bestest Jedi Evar to make up for his horrible choices in the original timeline, I'm pretty sure it would be a lot more likely for him to, I dunno, try and kill Palpatine and most of his henchmen. Or do something stupidly and shortsightedly violent and damn the consequences. It could still be all angsty and stuff! Think about how Obi-Wan and the Council would react! Think about how the Senate (which Palpatine has already been priming to distrust Jedi) would react to a Jedi killing the Chancellor!