Know the Demand
You must know how many users do you expect everyday and what kind of capacity you will need to build. Are you going to be catering to a few dozen enthusiasts a day or will it be hundreds coming in to hone their skills. This is important to know as it will allow you to develop your infrastructure accordingly. You do not want to have too much capacity when the audience is little and do not want to lose customers because the capacity is just not enough.
Type of Sports
Each sport has its own particular requirements. You need to be aware whether you will be having more BMX bikers or skateboarders. You are likely going to have a mix but you need to know that which type of sport is preferred more and must build a greater capacity for that sport.
You also need to know about the likes and dislikes of those who will be coming to your park. This is crucial as small flaws in design can leave you with a limited attendance. You can conduct a survey about these preferences and you can also visit the online forums for these sports to see what is liked and disliked. Be sure to look at the suggestions these enthusiasts have to improve the quality of existing facilities.
Make sure that during the whole process, you do engage professionals and the final design will have to be made by a professional architect. This will allow you to have an optimum utilisation of space and a design that will be as per the current standards.