As a lawyer, i was very interested in this thread.
The way I see it is that if these two things happen:
1 - No money is involved.
2 - No posing as new version or as the original.
it's very likely that you will never have problems or, at most, you will be asked to take your project down with no other demands.
From what I read from both sources, all cases where people got sued were deviations from what a mod is, which, if i understand correctly, is:
A modification made to a game (with tools provided by the game itself or not) to attain effects that the unmodded game didn't have or couldn't do.
Also, a clone game could can be defined as this:
A game which is intrinsically identical to the other one, which includes more than main elements (usually a judge decides the criteria used to define closeness and other ways of finding if there is a copy and who copied who)
Both definitions can intersect as one can argue that with too much modifications a mod becomes a new entity, different than the original (It's the same principle we used for songs which use samples from other songs)
SO, if you do not monetize, do not use direct assets and do not pose as the original, you will have a rock solid defense in case someone get angry and try to sue you for your mod.
In my experience as a lawyer I've seen a lot of people suing for stupid things and getting absolutely nothing. My colleagues that work with intellectual properties are adamant that only when things are glaringly copied or when two big companies are fighting it out on court is that they actually do something, the rest are not even good cases.
Finally, i'd like to thanks both SteamUser and Blazed, this heated discussion was bad in a sense, but it would be interesting to have all of their tips stickied in a thread, like DO's and DONT's of MODDING.
