This question is about the Warhammer 40K LCG, but we don't currently have a tag for that, and I wasn't sure how we wanted the tag to look. Should it just be warhammer-40k-conquest or do people have other ideas that might work better? I'm a little concerned that is too easy to confuse with normal Warhammer 40K.
I recommend warhammer-40k-conquest to match the proper name of the game. To my knowledge, no other Warhammer 40K product has a name that is close to it, so the chance of false tags is unlikely. We can create a tag synonym warhammer-40k-lcg if desired to match the format of star-wars-lcg and the tags for other Fantasy Flight Living Card Games, but I think that the full name tag should be the primary one.
Also, we ought to make sure that any new tags include a tag excerpt to say that it is an LCG, rather than leaving it blank. That way, users typing in the tag will see that this refers to the LCG and hopefully pick a different tag if they didn't intend for it. I suggest the following:
A Living Card Game (LCG) produced by Fantasy Flight Games set in the Warhammer 40K universe.
Please note that as of the writing of this answer, there is a tag currently in use for Warhammer 40,000: Conquest; it is conquest-lcg. (Not what I would have created, mind you, but there it is.)
I'm the one that created the conquest-lcg tag for my own question.
I am not married to this tag, but I also don't think it needs to be renamed as, at this stage, there is not much else it can be confused with.