stop dreaming bro
Nothing wrong with dreaming.
This is entirely possible...
However, even if you did get everything (functions, assets, maps, etc...) it would still feel like a different game. Engine dynamics play a big role. Slightly different networking code would be enough to drive hardcore dedicated players away. It would be a huge effort and even if it was popular, you'd still have to deal with the fact that you're ripping content from a game without permission. Yes, we do do that here but taking from AA 2.8.5 or AA3 and implementing into 2.5 and keeping the same gameplay, it's kind of a grey area.