I think the best approach would be to modify not just the resolution that the game runs at, but also the viewport and projection / field of view used by the game such that you effectively increase the viewable area beyond what was originally possible.
This could be a potential concern in games with competitive multiplayer modes. However, I don't recall Sacred having one of those.
I can't really tell you how to accomplish this specifically, since it will depend heavily on how Sacred itself was built. In general you are going to want to find where the resolution is set and modify that, and also where the viewport / projection matrices are defined and adjust those. You will likely need to adjust the positioning of various UI elements, and so on.
This appears to be the approach taken by the widescreen mod you linked, at least, and possible the D2 one as well (I didn't look at that one in as much detail).
However, while the above could be quiet a lot of work it's probably better than your other options, which are:
- Allow the textures to stretch, use a better-quality filtering method to hide the artifacts as much as possible (probably not what you want at all).
- Re-author the textures are a correspondingly-higher resolution. Not only is this a lot of artistic effort, I have strong reservations about suggesting it because of the potential increased risk of IP law complications.