Well, the flashback takes place sometime shortly before Episode II, so they could potentially cover a story a year or two before Episode II, where they both look like that and before the war began (I think Outbound Flight would fit in this time period). You couldn’t go too far back though, or Anakin would be need to be younger, which Hayden couldn’t do now. Even though the Clone Wars show covers the time period between Episodes II and III, they could still do a live-action show about some untold adventures during that time. That seems like a no brainer to me. Honestly I expected to see a flashback of one of their adventures during the Clone Wars.

I don’t think there has been any canon story that covers the very early days of the war, when Anakin still had his padawan braid. This had been covered to some extent in the 2D animated Clone Wars micro series, and some of the comics, but they could tell a new story during that time.