The Beastmaster: Better fight scenes (the ones in the original looked choreographed), more nudity/sex (like the Juns not just slaughtering the people of Emir, but also having their way with their women), and cut the man-eating-bird-people out completely. Rewrite the ending so that it doesn't feel like a deus ex machina (spelling?). I'd have the city gates open after Dar defeats the leader of The Juns and the townspeople come charging out, weapons bared, to protect their young king. The Juns, who're leaderless and have lost many men to the flaming moat, lose moral and flee after a short fight.
Dungeons & Dragons: Have a script written by someone who had actually played the game. Don't use dragons as flying fire breathing horses, or beholders as multi-eyed guard dogs. Dragons also shouldn't so willingly accept being controlled by a magic scepter (this should really piss any dragon off). Maybe keep the same characters, but write a different story for the adventure.
Dungeons & Dragons II: Same story, but use different (better known) actors, better wardrobe (Lux didn't look very much like a barbarian at all), and better special effects. Add Norda the elven ranger and Elwood the dwarven fighter from the first movie to the party. Change Nim from a human to a halfling and get either Warwick Davis, Verne Troyer, or Debbie Lee Carrington to play the role (that last one will require Nim to be rewritten as a female halfling). I'd also change Berek the party leader from a fighter to a paladin. And add the scene where he finds the magic sword (though if he were a paladin I'd make the sword a Holy Avenger, and give the Vorpla Sword to Lux).
Conan The Barbarian: While I love the Schwarzenegger version, it was meant to be the first installment of a trilogy. With Ahnuld retired, for the trilogy to be completed the first two installments need to be remade. The story would stay basically the same, but I'd add some eliments from the novelization of the film. Things like... Conan's sword is never broken, he recovers it from Thulsa Doom's harem. The Eye Of The Serpent has mystical powers which allow Thulsa Doom to control his followers. The shaman has far more magical powers than what's shown in the movie, making him far more effective in The Battle Of The Mounds. King Osric is murdered by his guards. The skeleton in the crypt Conan hides from the wolves in comes to life and attacks him. Conan doesn't need a rope to climb The Tower Of The Serpent. The forging of his father's sword takes longer, with more dialogue between Conan and his father. The barbarians in Conan's village fare better against the Vaniir than they do in the movie. The Vaniir don't just slaughter all the adults, they also have their way with the women of the village. Conan's mother kills her fair share of Vaniir as well. Conan should be played by Tyler Mane, Valeria by Uma Thurman, and Subotai by Al Leeong. Thulsa Doom should be played by iether Wesley Snipes or Ving Rhames.
Canan The Destroyer: Keep the title, change everything else. Conan should be played by Tyler Mane again. If they bring back Malek from the original, make him brave and intelligent or make him mute. And if Queen Teramis is going to be the villain, than use the actual story from the Conan stories. Queen Taramis is abducted and replaced by her evil twin sister. The story is about freeing the real Taramis and getting her back in power.