I'm interested if it's possible to replace the HUD objects (like weapons) with 3D objects that would actually be in the game world?
This would especially improve the way melee weapons work.
Is it possible to achieve, and if yes, would anyone have any tips on how to start?

Hello
yes this is possible with a little app i made https://gamecreator.store/product/36249
i am not pushing my app but just informing you it is available, i get a lot of flak for advertising my apps
I can add more icons if some kind person would like to make some

Sounds like he's implying the weapons held by the player aren't 3D weapons... they are, they're just treated a bit like 2.5D
Melee really needs more animation variation and it'll be a lot better already

AHhhh ok my mistake

Yes, currently weapons are rendered as HUD objects, which is fine with firearms. On the other side, it makes it really hard to hit objects with melee weapons, so that's why I'm kind of interested if it would be possible to add the melee weapons as 3d objects, instead of HUD objects.
Of course, that would require scripting of some sorts (if it's possible at all).

yh because melee is treated like a gun is in GG - i.e. you hit whatever is in the exact centre of the screen once when the weapon is fired/swung... you could add more raycasts to increase the area it checks and / or add more rays on a delay during the swing to be more lenient on the hit detection but you are correct in that this would need to be done manually by code and also managing the hit impact damage etc.

This is one of my own 3d melee weapons (axe), so, yes, you can add your own 3d melee weapon, there is a guide in the GGMax root folder.
About stock melee, better raise a feature request on GitHub.

There is no physics applied to the actual weapon models. The weapon's physics forces are only applied when the weapon is used. For Bullets and Melee impacts. But I know why you raise this question as I have over a decade ago for the same reason.

Melee weapons in FPSC/GGC/GGMax are basically guns without flak visual effects. Swinging a sword to the right will not knock a basket to the right. In many games this works, but that's because of the way their engines handle the physics force directions based off the animations. It would be a neat feature to have this controlled in some sort of weapon editor.

Though about your question about physical first person 3D player weapons with collisions. This might be possible. Though I personally haven't used Max enough to know if that is supported, where you can float an object, attached to the player camera and have it maintain physics without gravity without jitter while in motion.

Another thing I'm unsure of in regards to entities, is if animations affect physics in any way or if their collision shape just stays still despite the animation showing movement. If that is the case, you'd have to entirely animate your melee weapon in Lua with motion and articulation for the swings.

The current weapon system is still based off the 17 year old FPSC way of handling melee attacks which is outdated by modern standards (technically outdated more than 15 years ago).