First off the Wookie Planet. I understand the technology aspect Lucas was shooting for, the David vs. Goliath, but if Endor would have been Kashykk, ROTJ would never get labeled 'kiddie'. I just watch the Ewoks now, and though Jar Jar is unbearable in TPM, the Ewoks hurt the seriousness of the movie. A really cool Wookie battle against the Stormtroopers would have made ROTJ a much darker movie that it needed to be.
The second death star isn't bad, though it is not creative like ESB. ESB showed us everything we never saw: Hoth, Dagobah, and Bespin. ROTJ showed everything a second time: Tatooine, Death Star II, and though we never saw Endor, the woodsy terrain was very dull. I would have liked to see them invade the Emperor's home planet, like some original ideas. Have the spacebattle with Han, Leia, Chewy, and the Droids, and have Lando in another ship attacking the blockade over Coruscant, that would have been a spectacular space battle.
You could still have Luke surrender to Vader, and still have the great scene between Emperor/Vader/ and Luke, but do it on The Emperors turf, maybe over a sea of lava. How ironic would it be for Vader to throw The Emperor into Lava when that is what almost did him in against Kenobi? Now some of these ideas probably couldn't have been achieved to due the special effects at the time, but who knows Lucas & Co. were pretty damn creative.
The other aspect is the long Jabba sequence in ROTJ. I loved this part as a kid, but now it does have its moments, but I think it goes on way too long. I always thought of Jabbas palace like a King, where you can't just walk in off the street. True they all had disguises to get in, but I would have like to seen them travel a couple of floors to get to Jabba, put some suspense in it. You can still have the rancor, as maybe Luke falls through a trap door, but keep everyone moving, that is why it is dull at some points, because everything is at a standstill. Also make it harder to save Han, maybe Lando get injured in the process. You can also still do the Sail Barge because Luke, Leia, and Han, always get caught, but in true Star Wars form, they always survive.
Well, that is what I think could have made ROTJ better, but I guess we have what we have. Oh well.