Depends where you live. A lot of countries have laws protecting derivative work in one context or another, but not all.
I am not sure, but i think derivative work follow a different pattern is not like Patent Law, wherein obvious variations of a method, system or product would be covered. Everything that we create is based on something else, even EU was based on other games, the art work basically need to be created from the ground up of course you can simply do subtle changes to not "infringe" on another person work, and you are not allowed to use copyrighted material such as names or tiles, such as XCOM or Muton, or the specific art wor, such as song images and such, in the case of images if you create your own images then you are not violating the right of the original creator.
However i am not your IP advisor nor do i intend to be I am just a simple forum lurker.
EDIT:
Also there is one more thing that i forgot, copyright is a little more abstract and the shades of gray exponentially increase, for example a case just finished over the summer, Apple vs Samsung, there were a few patent involved but there was also a design issue, Apple argued that Samsung had copied the "look and feel" of their OS, to cut the story short, Apple won, so even if you were to implement all brand new art and sprites and logo you could still be found liable due to following the exact same methodology. So in order to prevent that an overhaul of all the GUIs would be required, of course if the elements are not hardcoded then remapping them would be simple, as such in this case implementation will be different such that an old XCOM vet can see XCOM DNA in the game but it will not be confused with XCOM, anyway, i think i am going over board.
