I think it's pretty clearly going to be either KOTOR, or post ROTJ series with the Big 3 showing what happens in between Episodes 6 and 7.
I suppose the oft rumored Dark Times series (in between PT and OT) could happen. I get the sense though that Lucasfilm is deliberately trying to move away from "prequel era".
Here's a crazy idea though... what if they essentially recreate the Clone Wars but in the OT, by having something like REBEL WARS set in between Episodes 4 and 5? They could fit in a few seasons worth of battles or stories. Would be a good way to ease the world back into thinking of SW as the OT (leading up to the ST).