This is now your game. If you think you can remain faithful to the spirit of X-COM while introducing new varieties of aliens, go for it. Like the producers of Sherlock, I consider "canon" something we as a species build, not an untouchable perfect past state. That said, I would like this game to be "X-COM meets fantasy world". I'd rather They were still aliens - all that changes is that We are fantasy races. An important part of X-COM is the "weird other" feeling. I'm game with the idea that they attempt to infiltrate each society with their thin men as appropriate, and if you want to surprise us, I'm sure you can dig up obscure enemies from the sequels and what-not, but I'd rather you didn't go too far in the blending of the aliens with the standard fare of D&D.