Bear in mind that, once this meeting has decided a priority list, it then has to go the world governments for funding. There is no point in coming up with a scientifically wonderful proposal that you cannot sell to the politicians. Something exciting, thrilling, and easy to explain to numb-skull politicos is what you need.
Secondly, you have to ensure that virtually everyone is happy with the compromise ranking you come up with. All it takes in one irate astronomer to go to their congressional representative and tell them that the proposal is a big mistake, and you can say goodbye to your funding for any of these projects. Politicians (unreasonably) require that the astronomical community be completely uniform in its proferred views, before they will give us any money.
You have two personal biasses, and if possible, you should steer the meeting towards a conclusion that reflects these biasses. Firstly, history shows over and over again that the biggest results from any new telescope or facility are not the ones that were expected when it was built. Most of these facilities will not see first light for years: by then, the whole field of astronomy will have moved on. You have to make sure that whatever is funded has enough flexibility to cope with this.
Secondly, progress in astronomy depends on having a wide range of intelligent and imaginative astronomers. You have to make sure that these facilities can be used by young researchers, researchers at minor institutions, and from third world countries. And you have to make sure that people with unfashionable, maverick ideas, can get time to try them out. If, for example, you only had one, highly over-subscribed mega-facility, most of the time will tend to go to people with political clout, doing projects which conform to the current orthodoxy, and which have a very high probability of success. Original science, and new researchers might be squeezed out.