If I were to purchase a domain like: NYC.guide and there is a company that exists that is named NYC Guide would there be any possible legal issues?
no you'd be good to go. Don't see a trademark for it. And if your worried about the .com, looks like it hasn't seen content in years lol. The guys who are registering infringing new gtlds are pretty blatant w/ it. At least from what i've seen, some are pretty creative. Basically go way out of their way to do it....now all you need to do is wait for the release.....
not sure if you were just using it as an example, but it looks like .nyc is going to be released fairly soon. May want to consider hopping on both of those gtlds.
it's still to be announced. it's a pretty lengthy process to get everything approved and delegated, so you never really know...until the registrar knows for sure lol.