If you read earlier posts in this thread, 12 BBY seemed to be the most optimal year.
Clone Wars ended in 19 BBY, and so the first few years after that would simply be CIS Remnants rather than any flavor of Rebel Alliance.
"The Rebellion" as we know it became official in 3 BBY, and so we would probably legitimately see real "Rebel Alliance"-esque cells sometime before that. The game Force Unleashed deals with this time period the best. Jedi General Rahm Kota was launching raids against the Empire presumably as far back as 5 or 6 BBY in order to force a confrontation with Darth Vader.
If we simply did a median year, from 19 BBY to 0 BBY, it would be 9.5 BBY -- basically in an Episode III.5 sort of timeframe. This was why 9 BBY was my original proposal.
However, canon events closer to 12 BBY would seem more realistic considering this is when the Empire began integrating non-clones into the ranks in order to keep up with the manpower demands of expanding and maintaining Imperial dominance. So, tons more subjugation, oppression, and propaganda would've hit full-steam around this time I imagine.
My proposal is to go with 12 BBY, this seems to be a more "seed-planting" era for the proto-Rebellion to begin. 9 BBY might seem like a good compromise since it is the median year in the Rise of the Empire period, but it really depends on how "Rebel" we want the sim to be.
