In reply to:

But a prequel is actually a sequel in disguise. So prequel is, in actuality, inaccurate. A cheap cop-out, even.

So perhaps a sequel that explores exposition to the orignal film could be called a paleosequel.

pale-; paleo-; palae-; palaeo-: involving or dealing with ancient forms or conditions

Wellt did happen a long, long time ago in a galaxy far, far away...