Will we be getting Custom Vehicle support? If modders provide the parts?

Yes, that's what I am planning on!
The next releases will include a decompilable version of the main framework, and every of the vanilla vehicles will be decompilable so people can learn how to use the frame work and make their own vehicles.

Will we be getting Custom Vehicle support? If modders provide the parts?

I am not 100% positive it is impossible, I'm not sure if the animation is what causes the grace or some hardcode to be honest, I'll check it out when I get some time

Hello, and welcome! I really hope this doesn't come as demotivating & upsetting, but a lot of things in the list are unachievable at the moment; I'll list them below:

• god hand like fighting
  
• NPCs that fight back
  
• boss NPCs (they are to a limited extend)
  
• ground pound attack makes an explosion
  
• melee attacks can launch vehicles (you could maybe make a vehicle super weak to achieve it)

The rest are a somewhat not that hard to do however!
That sounds like one cool mod to be honest though

This mod has been updated & version 2.0 is now available.
This version features support for upcoming vehicle addons as well as a download link for a Red Ferrini addon and a new preset feature for the vehicles (2 at the moment).
Please check the description for more details!

Hey, I'm sorry to reply so late!
I think the easiest way of creating a home-made TAS is by making a macro that would simply play input for you, however it wouldn't be too useful as certain stuff (eg. AI) is not consistent.
If you're good at programming, you could maybe use a JavaScript API made by Tavis 'taviso' Ormandy that your program could request.