I think a series with background characters would work just fine, since they would cease to be just background characters as we got to know them through the course of the series, the main cast from the OT would end up the background characters for this series. What makes me iffy on this idea is that Lucas is writing the scripts himself is he not? If he's actually accepting some help and suggestions this time around or outlining the ideas to a writing team and letting them iron them out then I can see this working, but if not, we may get lots of great dialogue like the romance from AOTC. Lucas is a great idea man, but I'm not so sure about his writing skills.
I do hope this turns out someday, though I'd almost like to see the whole premise scrapped though and replaced with something completely different. It always seems to be the tiny timeframe starting with the PT and ending with the OT, why not do something fresh. Whole new time period, whole new cast, the works. Why not instead of a long time ago in a galaxy far away they make it a long long long long time ago in a galaxy far away or fairly recently in a galaxy far away. That'd be neat, and less likely to retcon a bunch of events from the films.