Formulate learning outcomes. Learning outcomes form the basis for the development of education and testing. A learning outcome is a description of what the student can do as a result of what has been learned.
In the design you also determine the assessment: how do students complete the open online course and does it assess the learning outcomes you have determined? It is also important to think about whether and how this online course is part of the curriculum. Of course, when determining and designing the assessment, the form should be taken into account: online and suitable for large groups.
When designing the learning activities, you take into account the form: online and suitable for large groups. There are of course some limitations compared to face-to-face teaching, but well-designed online learning activities, such as exercises, quiz questions, polls, discussions and self-assessment questions, can provide an effective way of learning. Take a look at this page of edX about possible exersices and tools.
Did you know that in addition to using video, you could also use audio only? A lot faster to make and sometimes just as effective for students.
Think about how interaction is part of the course: where can students ask their questions and when and how often do they receive feedback?
You are probably already an expert on the subject of the course, but it is always wise to make sure you have the most recent and up-to-date sources. So: take another good look at the sources you already have and possibly collect new ones. Consider the use of Open Educational Resources in the course. It can save costs and time. Of course, you can contact the accountmanager of the library if you need support in searching for and finding OER. It can also be very useful to cooperate with colleagues who are connected to the topic. You could use them as a source to write (parts of) the course or to critically assess the course.
Make sure you always mention a good source and also mention the (Creative Commons) rights of the course you have developed.