A game like Wolf:ET for example. For starters I assume all the models would have to be redone at a higher poly count using more sophisticated modelling an animation software, with better lighting effects than they had back 10-15 years ago?

I take it the rendering engine would have to be overhauled if you did this?

How about integrating say Bullet physics into the game, again I assume you just couldn't do this with the current engine?

What could you actually do to completely overhaul the game, graphics, particles, physics, maybe adding vehicles, planes etc..

titanfall was done with the half life engine and that is based on the quake engine. that's the scale you can achieve its just a matter of investment (time, money, knowhow).

it also depends on what side you want to invest, you could redo lot of art (most) and make it look way better. you could also redo all tech. and adding vehicles etc. would be rather a challenge on gameshow and network side.