If this is a thread about a power assisted 700 pound trailer, that isn't even really on topic. This is human powered site. I don't conceptually see anything wrong with one of the heavier end trailers, like the burning man or hobo types, being slowed to a snails pace in fine weather is not what anyone around here has in mind. But there are situations where it might make sense. But if it isn't human powered, then it is another forum, ATMO. You also have to be very careful. You would be way over the limit for a lot of components. You are going to start to need trailer brakes, and real serious brakes on the bike. It just ends up not being a bike bin kind of project, something more in the range of a scooter, and even then I don't think they are designed around that kind of load.