I've always felt that a DQ rep has been one of the most slept on choices for a smash character due to its insane popularity in Japan. I think the main reason we haven't received one is because it is difficult to determine if slime or one of the heroes should be playable. I personally believe that slime should be the playable representative as he is the most recognizable character in DQ, and has the most potential for an interesting moveset that properly represents DQ.
I see slime as a character that mostly draws off the Rocket Slime game for his moveset. In this game, slime (or Rocket, as he was named in this game) had to depend on picking up things to attack his opponents. The only attack he had when he did not pick up an object was to stretch himself and hit his enemies, which could be his side b and work the same way in Ultimate. This is why I feel that his neutral b should be picking up a random monster (out of 3: knight, platypunk, and dracky) to actually give him a way to attack. In Rocket Slime, when slime picked up a knight, he essentially became a slime knight and was able to attack with a sword.
Since he lacks limbs, I feel that he should play like Olimar and revolve around his neutral b. Without a monster on his back, slime cannot attack with most aerials and tilts, just like Olimar. However, depending on the monster slime has on his back, his aerials and tilts can be affected accordingly. For example, the knight can give him short range but powerful tilts and aerials, while the platypunk can give him longer range but weaker tilts and aerials, and the dracky would be somewhere in between. Maybe the neutral b can stack just like in Rocket Slime (with 3 monsters on slime's back) but only the one on the bottom is active. This way, the player can see which monster they have next if their opponent knocks off the current monster on their head after a certain amount of hits. Down b can work as a throw to get rid of the monster that slime currently has on his head. This move can work as a projectile and way to cycle to a monster that could have better aerials for the situation. Up b can be chimera wing or cannon, and the distance can be affected by the monster active on his head (knight being the shortest, platypunk being mid, and dracky being the best recovery).
TL;DR: Slime should reflect his moveset in Rocket Slime, which would play similarly to Olimar.