Make it Convenient
If the kitchen is the heart of the home, the game room is the place where people congregate -- after they finish eating. It'll become guest central in no time, and having a bathroom nearby will make life easier on all concerned. Adding a wet bar, or at least access to a mini fridge and a microwave, will keep the action centralized and save you from schlepping up and downstairs from the basement (or wherever) to make popcorn and get beverages. Game rooms are fun rooms, and yours will be much more fun for you if it's close to or contains popular amenities and features.