I'm thinking about building a home gym, what are some basic necessities?

Hate not having natural lighting. What about a skylight? I'm assuming your free weights include dumbells and interchangeable weight bars? I would suggest adding a "Total Gym" system for weight resistance training. Very compact. Very affordable. Very low maintenance! My physical therapist has me use one to rehab my shoulder - it's great! You could spend more money and buy the Bow Flex system. You do seem to be missing that aspect. I'd ad resistance bands for toning and a "bosu" ball for balance training with your free weights and turn it over and then do push ups, even do side plank push ups off the elbow/forearm. Include a round medium sized stability ball for core work and shoulder/back work. Essential.