Also in reality:
D3 has a bunch of weaknesses - no safe offensive options, easy to combo due to size, lack of non-read-based kills after 90%,
and difficulty against "lame" zoning play. Any buffs should directly address these points. Some hitboxes need fixing - Usmash and Fair specifically - and it'd be awesome if Fair/Bair had more of a hitbox closer to D3. Various frame data improvements would be awesome too (our Jabs seriously need help). But otherwise, three of my main ideas:
1. To scare opponents who normally would just shield everything: a killthrow/kill setup from throw a la Donkey Kong. IE: Dthrow -> Uair connecting til kill %s. This alone would do wonders for the character.
2. Fair to come out faster, have less endlag, and autocancel on shorthop. As it is, D3 has a huge weak zone in the air in front of him. If we could suddenly threaten with a fast, safe, and disjointed move (like Ike) it would be huge.
3. Gordo's reflect threshold to be raised to 6%+. The reflect mechanic is really interesting, but it needs to favor D3 more. A random Falco laser shouldn't hit the Gordo back. Opponents should need to invest just as much as D3 does percentagewise with the reflect game. This would be huge for some matchups like Duck Hunt, where Gordo would just eat the projectiles, while in no way being an untouchable barrier (it's so easy to shield/dodge through it already). But this would force opponents to respect D3 at distance and think more carefully about how they might approach - IE what every mid+ tier matchup is already.
We can look for a Jet Hammer buff, but honestly the move is just terrible overall and I'd rather have something like a killthrow setup. A guaranteed followup after Inhale would also be great, but same answer as before.