The graphics and battle animation are better than the first Goemon RPG, but the gameplay is the same old shit. The 3rd party member is Koryuuta, the blue haired girl who can transform into a dragon. I know she appears in the N64 games as well, but this seems to be her first appearance.
The 4th party member slot gets temporarily filled by various other characters you meet throughout the story. At one point you join up with Simon Belmont, and fight Dracula in his castle... seriously. Another person who joins you is Nana-chan, who you meet in some random underground passage, and when you infiltrate some castle and take out the boss in it, she reveals herself to be the true princess of that castle.
Then there's this part where Goemon's group falls off a bridge, and they fall a long way and end up in some strange, trippy world where everything is made of food.. this seems to be a precursor to all the food-related stages in later Goemon games. Then there's a giant butterfly that picks us up and takes us to some other world that's like the food world, but with a pallet swap, and seems to be about toys and monsters rather than food...
Another party member who joins you is a ninja woman named Haduki. She seems to be going after the same enemy we are.. then there's a part where they go to the moon. There's a village of rabbits, and an alien joins your team for just one dungeon before dropping you off back on earth with his UFO. Haduki rejoins you near the end to take on the final boss.