Ah the possibilities ...
yes, I know what you mean Bingo, but it could turn out in different ways:
- the plan is to capture the Emperor or invade Coruscant in a desperate strike to end the civil war (in which Palpatine traps the rebel fleet)
- our heroes (after or instead of Han's rescue) steal the protoype* to help the trapped fleet (some tweaking needs to be done here)
- no prototype side-story / heroes meet with fleet, mostly happens as in original, only that there are more DS / Palpatine crushes his own ships / DS#2 or SuperStarDestroyer moves against other DS (with Emperor on it)
I see some options as to what and why - Palpatine sacrificing his troops (the Superlaser could blow up ships or he could order to shoot at Endor, as was considered - therefore driving some Admirals and Captains to defect/refuse to obey orders)
* I think the prototyp more of the basic parts, the generator(core) and the attached Superlaser - maybe some operational stations, but in general it should not be finished (maybe even the laser isn't tested yet and they ram the thing into the other like the SSD did)
add to "capture prototype": maybe the heroes try to infiltrate DS#2 instead of landing on Endor - so the "bait" could be used as suggested above to damage the finished DS and helping the rebel fleet (it could all happen in Coruscants orbit, so the end-battle isn't divided over half the galaxy)