I know that some time ago the AAA team mentioned the fact that they were planning to implement compatibility for custom gun and character skins, and I was curious to see if there is still any intention of doing so.

If yes, I'm also curious to know if custom models and sounds would be possible.  I think that certain people would be more attracted to the game if there were not only different paint jobs for custom guns and characters, but also completely different looks and sounds (maybe not completely different character sounds, but at least the ability to get a shotgun, etc. that sounds more to your own liking). 

It will be cool if you will can make your own character, by making face, choose your clothes etc. But in first game to have a option like "RANDOM". To be honest, I was really lazy to make my own character, first time when I played Skyrim xD.
Also, if we can make it like TF2, it will be cool if you can find different "dropped weapons" or "powers" for aliens.

At weapons, it will be cool if you can make it colors, add your own AIM etc.

It will not be there from beginning (probably), but yes, it is planned. You will be able to create own models, sounds, textures etc. for replacing default ones with same abilities and proportionals. But to do so, you will need external software (Photoshop, 3D modeller etc.) and Unreal Engine 4 for creating packages. Anyway, creating own 3D models is not easy, with all textures, sockets, animations etc. I think that common will be just modifying textures and changing sounds.

Customization directly in game like in Skyrim etc. will be not possible. Dropped weapons and powers could break balance and will not be there but later we will think about something like that - maybe some costume things.

There is also one chance for simple customization - to change color of your character/weapons. We have it into own suggestions, but still it is not planned. We will discuss it after release. It would be good to have ability to choose little different look in game without modifying some packages. Maybe this could be unlocked by those dropped things from enemies.