What legal issues could meet developer if he I'll try to use game design ideas on which was based free mod for Unreal?
IANAL, but generally I don't think pure game mechanics are copyrightable/patentable. This is why people can clone games with impunity, using the same mechanics, control scheme, etc. but putting a different name and different art, sound effects, etc. on it. So if you want to make a game with similar "design ideas" (I presume that means mechanics) to an existing game/mod, I think you're fine as long as you don't copy their name, art assets, and so on.
Thank you very much, Reedbeta! What problems could I meet if I'll use not only mechanics, but also art/sound/map design from free mod?
I've seen it done before, so it's not new particularly in the free mod community. Most are just learning or trying stuff out, so everybody just does whatever they want (not that that's a good thing). Just don't use any physical assets (code, sound, art) without permission, because that's grounds for IP theft and you could be chastised for that. It's better if you're original with such things. Don't try to copy because that's easier than thinking up something new.
Forward question, how I can know that mentioned code, sound, art is copyrighted?
In some situation it's impossible ask for permission - author isn't available online and I don't know what is his real name.
Probably I can try to discover his name cos I know name of his unfinished project which I'm interested to complete.
How I could investigate to whom this IP belongs? Via some official department?
Assume it's copyrighted until proven otherwise. In the US and indeed in the vast majority of all countries, legally you have a copyright automatically as soon as you create something. You do not need to register it or anything (although it's possible to do so as a matter of documenting your creation).
If you can't find the original author and/or the author does not grant you permission to use their work, you're outta luck. Although in practice, if it's a free mod they're probably not going to sue you, since suing costs money.
Thanx all. I'll build another clone.
This site may help answer some of your legal questions: http://www.newmediar...al_Property%20/