Personally, I hope the movie has a good component of diplomacy to it, perhaps taking place as a solo act during the Clone Wars. He did have the nickname of "The Negotiator", after all, and I feel like really bringing that to the forefront and showing him solving a genuinely complex, multi-sided problem would be interesting.
A Yoda movie? WTF?
I am probably in the minority here, but I like to think not much of consequence happens to Obi or Yoda in between ep.3 and ep.4. I mean obviously they live and probably cant accomplish anything too significant since most folks have all but forgotten them. Seems like some pretty difficult writing constraints to work under. You could go before the saga I suppose, but at this point I think they are just playing to fan service instead of pursuing original stories in the SW universe.
The Obi-Wan movie will probably be during the Clone Wars, but I could also see where he would have to go into action while on Tatooine in order protect Luke. Say pirates, slavers, dealing with an vengeance minded Watto, etc.
I see the Yoda movie happening 200-400 years before the prequel trilogy.