Some thoughts -
Re: timing - a weekend in early September could avoid the summer heat and traffic, and separate it from the large number of motorcycling events which take place earlier in the year?
Re: format - I know LE to JOG has been suggested but having done this a number of times (including taking a 1000 mile route, doing it without motorways, and completing it in 14 1/2 hours), I would suggest this is both uninteresting in terms of route unless taken over 3 or 4 days to take in some more challenging roads, and rather unimaginative as a challenge (a much better ride is to go from the eastern point of Britain to the most westerly point

I think it should aim to either be reflective of the history of the MCC, or something new to the club altogether, with the express aim of attracting new members.
Perhaps an example of the former might be to try and replay an historic MCC ride - maybe starting from the Ace Cafe and riding to Lands End and back, via a series of locations (which might include some easier tracks and sections?). An example of the second might be something similar to the Irish Photo Rally, where competitors are sent a number of locations to be visited and photographed. This could be over a short period of time with set start and finish, or over a number of months e.g. April to September so competitors can arrange their own timetable. Locations could include some places relevant to the MCC’s history. There could then be a social event at the end of the time period when competitors could meet and share displays of their photographs and stories.