How about this? - within the prologue of ROTJ, you could establish that much more time had passed, and the rebellion has gained a huge momentum, having recently captured Coruscant.

The Emperor and much of the Imperial fleet has since disappeared - however there are relentless attacks on the capitol by the Imperials.

The Rebel's desperate to hold the City Planet will have committed ALL their resources to holding it (all the eggs are in one basket).

The Rebel's discover the Death Star II plans within the city (opening scenes) - and thus a strike team is forced to chase it down. Meanwhile Luke/Leia etc are on tatooine freeing Han.

The big reveal would be that the EMPEROR is on the Death Star, and he is going to use it as his new seat of Power. He allowed the Rebel's to audaciously capture Coruscant in order to force them into one single location, which he intends to destroy.

It might up the stakes a bit when they realise it's operational, and at any given moment he's going to jump into Hyperspace.