Themes, entertainment and décor are important (and fun) aspects of any luncheon, but first things must come first: What are you realistically capable of doing?
Before getting into the details, figure out the practical guidelines -- those will steer the rest of your decisions and prevent any logistical nightmares that could torpedo your lovely gathering. These practical concerns include:
Budget: How much can you spend? Do you have the money for an outside venue and/or caterers, or will you host your luncheon at home and do all the cooking?
Space: Will this be a mingling luncheon, or do you expect your guests to be seated the entire time? If it's the former, you should have enough space for people to move around without bumping into tables, chairs and each other.
Food preparation: If you'll be doing the cooking, you'll need to work within the confines of your kitchen. Be realistic about your oven/cooktop space, and plan the hot/cold proportions of your menu accordingly.
Food restrictions: Do any of your guests have food allergies? Anybody vegetarian or vegan? If at all possible, make sure you have at least one option for everyone, and omit any foods that could lead to anaphylactic shock -- never a pleasant event at any meal.
Purpose: Is this a party just for fun? A baby shower? An awards luncheon? This will help you determine how formal you want your event to be and whether a fun theme would support or distract from the central focus. (For instance, "Mexican fiesta!" is entirely appropriate for a casual wedding shower but might seem out of place at a fancy awards lunch.)
With these practical concerns in mind, you're ready for the best part: the food. A luncheon isn't a luncheon without it!