Personally, I think neutral B should be the Normal Gun (assuming we're not going with Gattling). Wouldn't neutral A be better suited for something like the Knuckle Gun? I mean, the way that gun lifts you IN Custom Robo would help to make Ray's tilts make the most sense.
Proposed Moves:
Jab: Knuckle Gun, three hits, final hit has an up-and-away trajectory.
Up-Tilt: Launches opponent up and behind, so as to put the opponent above Ray.
Forward-Tilt: Pushes opponent slightly up and away, similar to Jab, but on a lower trajectory.
Down-Tilt: A kick (probably with a little rocket boost visual effect?) that lifts the opponent up and in front of Ray
Forward Smash: The Shotgun from Custom Robo, high damage, trajectory similar to Mario's.
Up Smash: Wall Pod-like explosion above Ray. Trajectory would be up and away.
Down Smash: A pod explosion that Ray quickly fires into the ground. Low trajectory and away.
Neutral Special: Normal or Gattling Gun: If you want the Normal Gun, you could tap B for a quick three-round burst with minimal damage and knockback, maybe get an aerial angle like Sheik with her needles, albeit with less range. OR, you could consider the Gattling Gun, and it would function similarly to Fox's.
Up-Special: The Umbrella Pod. Ray would float upwards with the pod exploding above him at the peak of the jump. Could function similarly to Snake's Cypher in that Ray would be protected throughout the recovery, and similarly to Sonic's in which Ray could attack out of it.
Side Special: Dolphin Pod (?). Would run a very short distance ahead of Ray and explode, dealing more damage with a direct hit on an opponent. High ending lag and some startup, so as to prevent Dolphin spamming.
Down Special: Acrobat Bomb: Would propel Ray a great height, similarly to Bowser's down-special. Deals little to no damage, but has a meteor-like trajectory in the air with a small hitbox.
And I can't be bothered to think of aerials at the moment.