I don't know exactly how many people are using Brilliant now, but I figure it must be getting fairly high, and I can't help feeling that whilst at the moment members are all working on different problems which aren't particularly linked together, it would be great to have some sort of polymath project on the go which anyone can contribute to as part of a larger investigation.
The benefits of the current format are obvious: there are problems and wiki posts from a range of topics and difficulties, which people can work through at their own speed and interest. However, a polymath project could draw from the combined knowledge of all of us. I am nowhere near as good at problem solving as some members, but the idea of these projects is that one contributes all they can and others build upon it. I have no idea what the investigation would be about, but I like to think there are some areas of maths to discover which are within reach if we combine our efforts. Imagine how awesome it would be (and what great attention it would bring to Brilliant) if a group of young people managed to have some fresh insight into a problem, or a different way of looking at something which others ignored.
Of course, a staff member would have to organise it, and maybe it'd all just be too tricky. Anyway, it would be good to get a sense of whether others would like to take part part in a polymath project. You can have a look at some others online to see some examples.