I'll start by stating that I'm mostly look for open minded discussion here. One of my biggest struggle with the D&D stat system is that flat out a 20 strength Goliath is equally as strong as a 20 strength Halfling. This often leads to weird interactions when it come to everything except combat.

For this specific point my idea/suggestion initially landed on changing the DC or things depending on the style of characters. For example say a Goliath or Firblog something naturally larger than common would require a slightly higher DC for stealth compare to say a Gnome or Hafling that is naturally smaller. This could be as simple as +/-5 to the dc depending if they are larger or smaller than medium.

Another topic is armor and how heavier armor doesn't really offer much in the sense of bonuses and instead hinder the players that go for that play style.

A fighter with a AC 18 wearing plate armor has disadvantage on stealth while only getting flat AC. A lot of classes can match that same AC naturally without the same sort of disadvantage applied.

One solution for this I was thinking would be heavy armor offering some damage reduction for attacks that do hit while working the same for anything that doesn't. After all a sword hit landing against an armored individual might not cut but would still have affect in the sense of blunt damage at that point. This would give the more armored classes stay relevant as the more magical ones gain all their cool features around them.

Being large wil lmake hiding more difficult, that's to be expected.

Goliaths and Firbolgs (and Orcs, oh my) all have Powerful Build, making them stronger than equal-strength halflings. As such, outside of combat, they can lift more (far more) than the equal-strength Halfling.

Armour I do agree with to some extent - but the idea is that the heavy armour character can dump dex without compromising their armour class, so they can be strong to swing their weapons, have high con to tank any hits that get through, and still have a decent AC. Rogues and Monks don't tend to put into Con, they go for Dex primarily to up their AC. Heavy armour frees up that stat.

I would like to see an overhaul of Armour. Damage reduction is going to scale poorly (low level, dropping 10 damage to 5 is huge. High level, dropping 100 damage to 95 is meh!), I would instead have heavy armour stack up more - Base Chainmail is AC 16, Plate is AC 18, Shield is +2, Heavy Shield is +3 and requires Str 16, Helmet is heavy only and gives disadvantage to Perception (-5 to passive) and +2 to AC. Pauldrons add +3 to the Strength requirement and a further +2. That way you can go all-in on heavy and end up with AC 25, which is not uncommon, but you've used no abilities except proficiency and Strength (which you are likely using anyway) to get there. You'll need lots of money and strength 19+ to do this.