There's a few utlities that can give you a map (at the world level) with customised terrain.
I saw
DwarfHeightMap (fairly old) and
PerfectWorld (fairly recently updated), when I had a quick search. I've never used anything like that, myself, to know if they still work but I've heard the latter mentioned more recently. That's if you need the entire world to sufficnetly reflect the Gothic II map (not familiar with its extent).
[Ninjaed, on that point...]
On the localmap scale, i.e. the ground-upon-which-you-will-adventure, if you need a lot more reflavouring I think you're normally more at the whim of how the world tile(s) then render into exact terrain, but I'm sure that with enough DFHack manipulations piped from suitable inspirational material you could tweak the land even more to your liking that if you or someone else has put enough effort into working out what to do to replicate it (
Quickfort might do much for you, or clue you in on what's actually needed).
Then I think you might need a smattering of modding to implement custom containers (to be placed by the above?) and anything else you think you need for your revamped immersion. There's a whole lot of people around these parts who can advise on such experiments (not me, so much).
How much of a project it'll be is, I think, up to you. It'll be a project and a half. But as an xkcd person I could imagine you being thoroughly nerdsniped by the implementation and having at least as much fun as any playthrough.