Well in my mind, this should've been in the PT. The PT should've started with Anakin and Obi-wan already as adults and Anakin ultimately falls to the darkside at the end of Ep.II (perhaps mirroring the downer nature of TESB). Ep.III should've been Obi-wan pursuing Anakin and trying to bring him back over several encounters, finally culminating in a heartbreaking duel where Anakin is presumably killed as far as the audience is concerned. Ideally that would've been how the PT played out and Obi-wan stays in hiding for 20 years, which I have no problem with. Heck in the OT, Yoda had no obvious way of even leaving the planet, although one deleted RotS scene did show Yoda once had some sort of weird pod thingy. Frankly, it would've made way more sense to me for Obi-wan to take one kid and Yoda taken the other, but obviously George had written that possibility out a loooong time ago.
But that didn't happen so despite my reservations about this show even happening, the one silver lining for me would've been if they did latch onto Vader's RoTJ line and Obi-wan attempts to do what I felt should've been in the PT: attempting to find Vader and bring him back. That would've been a credit to the canon and a worthwhile effort. Instead what we've gotten is a canon bending (if not breaking) thing that seems rather pointless.
Well I would've preferred Qui-gon been a supporting character in the show but I could see them mimicking RoTJ's ghost scene in his overdue appearance. I think his dialogue will be minimal, if at all.
This is what I thought the Prequels would be. Obi discovers young adult Anakin. Ep 2 ends with him becoming Vader and Ep 3 is Obi trying to stop and save him. Just like you said. That's what me and all my fellow fan friends thought would happen but obviously not.