Facilitating an online conversation between senior leaders and employees worldwide

I am in the process of designing an online event to facilitate a global dialogue between senior leaders and 3000+ employees. The online conversation will last 60-mins. Do you have experience hosting online conversation on your intranet? Do you have any tips/checklist/guidelines/philosophies/methodologieis which you find helpful? I understand that preparation is the key to a good online conversation event. To me, this means that I need to put in some effort and thoughts to design the communication procedure to allow genuine dialogue to happen, and to address any power issues which may silence some people and favour others.